* String interpolation. Expressions like

"--with-freetype2-library=" + freetype + "/lib"

  can now be written as

    "--with-freetype2-library=${freetype}/lib"

  An arbitrary expression can be enclosed within ${...}, not just
  identifiers.

* Escaping in string literals: \n, \r, \t interpreted as in C, any
  other character following \ is interpreted as-is.
  
* Newlines are now allowed in string literals.

tests/lang/eval-okay-string.exp

@@ -1 +1 @@
-Str("foobar/a/b/c/d/foo/xyzzy/foo.txt/../foo/x/y")
+Str("foobar/a/b/c/d/foo/xyzzy/foo.txt/../foo/x/yescape: \"quote\" \n \\end\nof\nlinefoobarblaat")

tests/lang/eval-okay-string.nix

@@ -1 +1,9 @@
-"foo" + "bar" + toString (/a/b + /c/d) + (/foo/bar + "/../xyzzy/." + "/foo.txt") + ("/../foo" + /x/y)
+"foo" + "bar"
+  + toString (/a/b + /c/d)
+  + (/foo/bar + "/../xyzzy/." + "/foo.txt")
+  + ("/../foo" + /x/y)
+  + "escape: \"quote\" \n \\"
+  + "end
+of
+line"
+  + "foo${if true then "b${"a" + "r"}" else "xyzzy"}blaat"