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
Merge pull request #22 from gifnksm/master

bigint: Remove `parse_bytes` method