Recent numerics stabilization removed the inherent `min_value` and
`max_value` methods from integer types, assuming that the module-level
constants would suffice. However, that failed to account for the use
case in FFI code when dealing with integer type aliases.
This commit reintroduces the methods as `#[stable]`, since this is
essential functionality for 1.0.
It's unfortunate to freeze these as methods, but when we can provide
inherent associated constants these methods can be deprecated.
c0f86a953c