Add a primop for regular expression pattern matching

The function ‘builtins.match’ takes a POSIX extended regular
expression and an arbitrary string. It returns ‘null’ if the string
does not match the regular expression. Otherwise, it returns a list
containing substring matches corresponding to parenthesis groups in
the regex. The regex must match the entire string (i.e. there is an
implied "^<pat>$" around the regex).  For example:

  match "foo" "foobar" => null
  match "foo" "foo" => []
  match "f(o+)(.*)" "foooobar" => ["oooo" "bar"]
  match "(.*/)?([^/]*)" "/dir/file.nix" => ["/dir/" "file.nix"]
  match "(.*/)?([^/]*)" "file.nix" => [null "file.nix"]

The following example finds all regular files with extension .nix or
.patch underneath the current directory:

  let

    findFiles = pat: dir: concatLists (mapAttrsToList (name: type:
      if type == "directory" then
        findFiles pat (dir + "/" + name)
      else if type == "regular" && match pat name != null then
        [(dir + "/" + name)]
      else []) (readDir dir));

  in findFiles ".*\\.(nix|patch)" (toString ./.)

tests/lang/eval-okay-regex-match.exp

@@ -0,0 +1 @@
+true

tests/lang/eval-okay-regex-match.nix

@@ -0,0 +1,26 @@
+with builtins;
+
+let
+
+  matches = pat: s: match pat s != null;
+
+  splitFN = match "((.*)/)?([^/]*)\\.(nix|cc)";
+
+in
+
+assert  matches "foobar" "foobar";
+assert  matches "fo*" "f";
+assert !matches "fo+" "f";
+assert  matches "fo*" "fo";
+assert  matches "fo*" "foo";
+assert  matches "fo+" "foo";
+assert  matches "fo{1,2}" "foo";
+assert !matches "fo{1,2}" "fooo";
+assert !matches "fo*" "foobar";
+
+assert match "(.*)\\.nix" "foobar.nix" == [ "foobar" ];
+
+assert splitFN "/path/to/foobar.nix" == [ "/path/to/" "/path/to" "foobar" "nix" ];
+assert splitFN "foobar.cc" == [ null null "foobar" "cc" ];
+
+true