parse_bytes has been removed, use from_str_radix instead

see rust-lang/rust#18536
Merge pull request #21 from japaric/from-str

parse_bytes has been removed, use from_str_radix instead
bigint: Refactor the `parse_bytes` and `from_str_radix`.

This follows the changes of "Separate string->integer implementation in strconv" (rust-lang/rust@138b76b).
This allows `parse_bytes` to be removed without breaking `libnum` code.
Use sliciing_syntax
Fix lint name warning