Add isascii to normalize_identifier for 25% reduction in parse time (#228)

Author: ndinsmore

src/literal_parsing.jl

@@ -353,5 +353,5 @@ end
 
 function normalize_identifier(str)
     flags = Base.Unicode.UTF8PROC_STABLE | Base.Unicode.UTF8PROC_COMPOSE
-    utf8proc_map(str, flags)
+    return isascii(str) ? str : utf8proc_map(str, flags)
 end