std: Add stability attributes to primitive numeric modules

[brson]

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 impls - Wrapper around fmt::radix. Wrong name (Str vs String)

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

[brson]

Also deprecates int::to_str_bytes, which is a strange function that formats a number without allocating, and is unused.