Deprecate free-standing endian conversions in favor of methods on Int. Merge Bitwise into Int and add more bit operations.

The free-standing endian conversion functions have been deprecated in favour of a group of conversion functions grouped together in the `Int` trait:

- `mem::from_be*` -> `Int::from_be`
- `mem::from_le*` -> `Int::from_le`
- `mem::to_be*` -> `Int::to_be`
- `mem::to_le*` -> `Int::to_le`

The `Bitwise` trait was also merged with `Int`. This further simplifies the trait hierarchy.

The `get_bit`, `set_bit`, `clear_bit`, and `flip_bit` methods were added to the `Int` trait.

[breaking-change]

`core::num::Bitwise::swap_bytes` was moved to `core::num::Int`

The free-standing endian conversion functions in `core::mem` have been deprecated and had their `#[stable]` attributes removed.