Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

tests/functional/lang/eval-okay-fromTOML.nix

@@ -0,0 +1,208 @@
+[
+
+  (builtins.fromTOML ''
+    # This is a TOML document.
+
+    title = "TOML Example"
+
+    [owner]
+    name = "Tom Preston-Werner"
+    #dob = 1979-05-27T07:32:00-08:00 # First class dates
+
+    [database]
+    server = "192.168.1.1"
+    ports = [ 8001, 8001, 8002 ]
+    connection_max = 5000
+    enabled = true
+
+    [servers]
+
+      # Indentation (tabs and/or spaces) is allowed but not required
+      [servers.alpha]
+      ip = "10.0.0.1"
+      dc = "eqdc10"
+
+      [servers.beta]
+      ip = "10.0.0.2"
+      dc = "eqdc10"
+
+    [clients]
+    data = [ ["gamma", "delta"], [1, 2] ]
+
+    # Line breaks are OK when inside arrays
+    hosts = [
+      "alpha",
+      "omega"
+    ]
+  '')
+
+  (builtins.fromTOML ''
+    key = "value"
+    bare_key = "value"
+    bare-key = "value"
+    1234 = "value"
+
+    "127.0.0.1" = "value"
+    "character encoding" = "value"
+    "ʎǝʞ" = "value"
+    'key2' = "value"
+    'quoted "value"' = "value"
+
+    name = "Orange"
+
+    physical.color = "orange"
+    physical.shape = "round"
+    site."google.com" = true
+
+    # This is legal according to the spec, but cpptoml doesn't handle it.
+    #a.b.c = 1
+    #a.d = 2
+
+    str = "I'm a string. \"You can quote me\". Name\tJos\u00E9\nLocation\tSF."
+
+    int1 = +99
+    int2 = 42
+    int3 = 0
+    int4 = -17
+    int5 = 1_000
+    int6 = 5_349_221
+    int7 = 1_2_3_4_5
+
+    hex1 = 0xDEADBEEF
+    hex2 = 0xdeadbeef
+    hex3 = 0xdead_beef
+
+    oct1 = 0o01234567
+    oct2 = 0o755
+
+    bin1 = 0b11010110
+
+    flt1 = +1.0
+    flt2 = 3.1415
+    flt3 = -0.01
+    flt4 = 5e+22
+    flt5 = 1e6
+    flt6 = -2E-2
+    flt7 = 6.626e-34
+    flt8 = 9_224_617.445_991_228_313
+
+    bool1 = true
+    bool2 = false
+
+    # FIXME: not supported because Nix doesn't have a date/time type.
+    #odt1 = 1979-05-27T07:32:00Z
+    #odt2 = 1979-05-27T00:32:00-07:00
+    #odt3 = 1979-05-27T00:32:00.999999-07:00
+    #odt4 = 1979-05-27 07:32:00Z
+    #ldt1 = 1979-05-27T07:32:00
+    #ldt2 = 1979-05-27T00:32:00.999999
+    #ld1 = 1979-05-27
+    #lt1 = 07:32:00
+    #lt2 = 00:32:00.999999
+
+    arr1 = [ 1, 2, 3 ]
+    arr2 = [ "red", "yellow", "green" ]
+    arr3 = [ [ 1, 2 ], [3, 4, 5] ]
+    arr4 = [ "all", 'strings', """are the same""", ''''type'''']
+    arr5 = [ [ 1, 2 ], ["a", "b", "c"] ]
+
+    arr7 = [
+      1, 2, 3
+    ]
+
+    arr8 = [
+      1,
+      2, # this is ok
+    ]
+
+    [table-1]
+    key1 = "some string"
+    key2 = 123
+
+
+    [table-2]
+    key1 = "another string"
+    key2 = 456
+
+    [dog."tater.man"]
+    type.name = "pug"
+
+    [a.b.c]
+    [ d.e.f ]
+    [ g .  h  . i ]
+    [ j . "ʞ" . 'l' ]
+    [x.y.z.w]
+
+    name = { first = "Tom", last = "Preston-Werner" }
+    point = { x = 1, y = 2 }
+    animal = { type.name = "pug" }
+
+    [[products]]
+    name = "Hammer"
+    sku = 738594937
+
+    [[products]]
+
+    [[products]]
+    name = "Nail"
+    sku = 284758393
+    color = "gray"
+
+    [[fruit]]
+      name = "apple"
+
+      [fruit.physical]
+        color = "red"
+        shape = "round"
+
+      [[fruit.variety]]
+        name = "red delicious"
+
+      [[fruit.variety]]
+        name = "granny smith"
+
+    [[fruit]]
+      name = "banana"
+
+      [[fruit.variety]]
+        name = "plantain"
+  '')
+
+  (builtins.fromTOML ''
+    [[package]]
+    name = "aho-corasick"
+    version = "0.6.4"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+    dependencies = [
+     "memchr 2.0.1 (registry+https://github.com/rust-lang/crates.io-index)",
+    ]
+
+    [[package]]
+    name = "ansi_term"
+    version = "0.9.0"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+
+    [[package]]
+    name = "atty"
+    version = "0.2.10"
+    source = "registry+https://github.com/rust-lang/crates.io-index"
+    dependencies = [
+     "libc 0.2.42 (registry+https://github.com/rust-lang/crates.io-index)",
+     "termion 1.5.1 (registry+https://github.com/rust-lang/crates.io-index)",
+     "winapi 0.3.5 (registry+https://github.com/rust-lang/crates.io-index)",
+    ]
+
+    [metadata]
+    "checksum aho-corasick 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)" = "d6531d44de723825aa81398a6415283229725a00fa30713812ab9323faa82fc4"
+    "checksum ansi_term 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ee49baf6cb617b853aa8d93bf420db2383fab46d314482ca2803b40d5fde979b"
+    "checksum ansi_term 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "23ac7c30002a5accbf7e8987d0632fa6de155b7c3d39d0067317a391e00a2ef6"
+    "checksum arrayvec 0.4.7 (registry+https://github.com/rust-lang/crates.io-index)" = "a1e964f9e24d588183fcb43503abda40d288c8657dfc27311516ce2f05675aef"
+  '')
+
+  (builtins.fromTOML ''
+    a = [[{ b = true }]]
+    c = [ [ { d = true } ] ]
+    e = [[123]]
+  '')
+
+]