Fix a small compile issue with latest Crystal version + Bug with non-ascii strings and comments

[anykeyh]

This fix two issues:

• Use as for forcing a type resolution that prevented compiling (one liner)

More important:

Fix non-ASCII chunks being truncated on load

Stack#run(String) passed buff.size (codepoint count) to
luaL_loadbufferx, which expects a byte length. Any chunk containing
multibyte UTF-8 was silently truncated by (bytes − codepoints), surfacing
as a spurious <eof> / unfinished string syntax error at the chunk
tail rather than at the offending character.

The fix was trivial:

Use #bytesize instead of #size.

I added regression specs covering Latin-1 supplement (é), Thai, CJK, em-dash, multibyte bytes inside string literals, and multibyte tails for good measure.

I wasn't able to run ameba, idk why but the binary wasn't compiling; although it should be fine in code-styling.

[veelenga]

Thanks