Add parsing related functions and traits.

[MashPlant]

In the str type, we have:

pub fn parse<F>(&self) -> Result<F, <F as FromStr>::Err>
where
    F: FromStr, 

Sometimes I just want to parse a u32 from a byte slice, but currently when working with std lib, I can only first perform checked convert from & [u8] it to & str, and then call str::parse. It would be a performance loss, because I don't really care whether this byte slice is a valid UTF-8 string.

Is it possible to add similiar functions and traits in bstr?

[BurntSushi]

This appears to be a duplicate of #64, which was created just a few hours before this one.