Refactoring, Code cleanup, etc.

Still cannot compile due to the loaning errors though.
Bump up version number to 0.2
Annotate FIXMEs to the blocker
Use parser.bump() in each_ltsv_field()

to skip TAB/LF.
Cleanup and refactoring
Use mutable field to avoid loan errors for now

Ideally, the parser could use inherited mutability, but
currently using &mut pointer causes confusing loan errors.