Add UTF-8 capabilities

[javierguerragiraldez]

Lua 5.3 already includes some:

• '\u{XX...}' embeds the UTF-8 encoding in string literals.
• %U in lua_pushfstring
• utf8 library (for codepoint handling, no Unicode semantics)

surprisingly, it seems it doesn't include

• %U in string.format

Other things that could be managed by a separate / optional library:

• conversion between different encodings. (windows still uses some mixture of UCS2 and UTF16)
• collation
• normalization, case folding
• text boundaries

The most obvious objection about including these capabilities with the language is the need of big tables. I think it would be valuable to evaluate what can the basic language do to make a binding as transparent as possible, without a hard dependency.