Proper parsers tests and examples for using `&str` as a parse source

[m4rw3r]

Problem

Currently all examples and tests are using byte-slices since that is most likely the most common source-input of a parser. But there are situations where an already valid UTF-8 string might need parsing, this requires some specialized parsers (the combinators should work fine), associated tests as well as examples on how to parse a &str.