Add utf8 support to tokenizer (#945)

* add utf8 support to tokenizer

* wrap utf8 functions in string table using a 'u' prefix

* document new utf8 functions

data/core/start.lua

@@ -35,6 +35,8 @@ table.unpack = table.unpack or unpack
 
 bit32 = bit32 or require "core.bit"
 
+require "core.utf8string"
+
 -- Because AppImages change the working directory before running the executable,
 -- we need to change it back to the original one.
 -- https://github.com/AppImage/AppImageKit/issues/172