objstr: Switch from in-object string data to ptr to separate memory area.

This is pre-requisite for having efficient implementation of str<->bytes
conversion, and having that efficient is required with unfortunare
str vs bytes dichotomy in Python3.
py: Make 'str' be a proper type, support standard constructor args.
py: Make 'bytes' be a proper type, support standard constructor args.
Merge pull request #355 from pfalcon/str-bytes-types

Make str and bytes be proper types