std: Add stability attributes to primitive numeric modules

The following are unstable:

- core::int, i8, i16, i32, i64
- core::uint, u8, u16, u32, u64
- core::int::{BITS, BYTES, MIN, MAX}, etc.
- std::int, i8, i16, i32, i64
- std::uint, u8, u16, u32, u64

The following are experimental:
- std::from_str::FromStr and impls - may need to return Result instead of Option
- std::int::parse_bytes, etc. - ditto
- std::num::FromStrRadix and impls - ditto
- std::num::from_str_radix - ditto

The following are deprecated:
- std::num::ToStrRadix and imples - Wrapper around fmt::radix. Wrong name (Str vs String)

See https://github.com/rust-lang/rust/wiki/Meeting-API-review-2014-06-23#uint