libexpr: Support structured error classes

While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.

tests/functional/fetchGit.sh

@@ -67,7 +67,7 @@ path2=$(nix eval --raw --expr "(builtins.fetchGit { url = file://$repo; rev = \"
 [[ $(nix eval --raw --expr "builtins.readFile (fetchGit { url = file://$repo; rev = \"$rev2\"; } + \"/hello\")") = world ]]
 
 # But without a hash, it fails
-expectStderr 1 nix eval --expr 'builtins.fetchGit "file:///foo"' | grepQuiet "'fetchGit' requires a locked input"
+expectStderr 1 nix eval --expr 'builtins.fetchGit "file:///foo"' | grepQuiet "fetchGit requires a locked input"
 
 # Fetch again. This should be cached.
 mv $repo ${repo}-tmp
@@ -208,7 +208,7 @@ path6=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; ur
 [[ $path3 = $path6 ]]
 [[ $(nix eval --impure --expr "(builtins.fetchTree { type = \"git\"; url = \"file://$TEST_ROOT/shallow\"; ref = \"dev\"; shallow = true; }).revCount or 123") == 123 ]]
 
-expectStderr 1 nix eval --expr 'builtins.fetchTree { type = "git"; url = "file:///foo"; }' | grepQuiet "'fetchTree' requires a locked input"
+expectStderr 1 nix eval --expr 'builtins.fetchTree { type = "git"; url = "file:///foo"; }' | grepQuiet "fetchTree requires a locked input"
 
 # Explicit ref = "HEAD" should work, and produce the same outPath as without ref
 path7=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = \"file://$repo\"; ref = \"HEAD\"; }).outPath")

tests/functional/lang/eval-fail-attr-name-type.err.exp

@@ -14,3 +14,8 @@ error:
             8|
 
        error: expected a string but found an integer: 1
+       at /pwd/lang/eval-fail-attr-name-type.nix:7:17:
+            6| in
+            7|   attrs.puppy.${key}
+             |                 ^
+            8|

tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp

@@ -5,4 +5,4 @@ error:
              | ^
             2|   key = "value"
 
-       error: while parsing a TOML string: Dates and times are not supported
+       error: while parsing TOML: Dates and times are not supported

tests/functional/lang/eval-fail-toJSON.err.exp

@@ -20,6 +20,11 @@ error:
             3|     true
 
        … while evaluating list element at index 3
+         at /pwd/lang/eval-fail-toJSON.nix:2:3:
+            1| builtins.toJSON {
+            2|   a.b = [
+             |   ^
+            3|     true
 
        … while evaluating attribute 'c'
          at /pwd/lang/eval-fail-toJSON.nix:7:7:

tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp

@@ -7,3 +7,8 @@ error:
             6|
 
        error: expected a string but found a set: { }
+       at /pwd/lang/eval-fail-using-set-as-attr-name.nix:5:10:
+            4| in
+            5|   attr.${key}
+             |          ^
+            6|