Change string to utf8 binary.

[Zachary Dean]

Changing the integer-list "string" to UTF-8 binaries internally will lower memory usage and extremely speedup text append operations (e.g: fn:concat).

• Change all string types to binary
• Change all built-in functions using xs:string to accept binary (string-length != byte-size).

[Zachary Dean]

Done with 08ebccc .