wasm: Parse and write JSON unicode strings.

This adds support for both UTF-8 and UTF-16. All the JSON value
strings use internal representation of UTF-8. Unicode validation and
translation to UTF-8 is performed only at the JSON parsing time.
There's no further encoding validation at the writing time but it is
assumed all the string operations maintain the validity of UTF-8
representation.

Fixes #1885

Signed-off-by: Teemu Koponen <koponen@styra.com>