Add FromBytesRadix trait + impl + error types

[alexjago]

This follows after discussion and work on #64.

Importing the trait FromBytesRadix will provide {integer}::from_bytes_radix(...) for integer types, analogous to {integer}::from_str_radix(...) but operating on an &dyn AsRef<[u8]> rather than an &str.

Error types ParseIntError and IntErrorKind are also provided, copying the standard library.
Note: IntErrorKind copies the post-1.55 version; once the crate MSRV is bumped it can probably be replaced with a simple pub use core::num::IntErrorKind.