Make primitive types that extend from Value be structs, and some errors when extending a struct from a class, etc.

Char#to_s works with multibyte chars. Fix String#replace, String#delete and String#each_char to work with multibyte characters.

Make sure to always raise Json::ParseException. Include line and column number in exception message.

When a macro’s body is a string interpolation whose pieces are always vars, don’t compile and execute its contents: do the interpolation manually.