adds support for unicode hexcode in string literals.

tests/lexer/tstrlits.nim

@@ -1,6 +1,6 @@
 discard """
   file: "tstrlits.nim"
-  output: "a\"\"long string\"\"\"\"\"abc\"def"
+  output: "a\"\"long string\"\"\"\"\"abc\"def_'2'●"
 """
 # Test the new different string literals
 
@@ -11,9 +11,13 @@ const
 
   raw = r"abc""def"
 
+  escaped = "\x5f'\50'\u25cf"
+
+
 stdout.write(rawQuote)
 stdout.write(tripleEmpty)
 stdout.write(raw)
+stdout.write(escaped)
 #OUT a""long string"""""abc"def