This patch does a few things:
- `__libcpp_is_signed_integer` and `__libcpp_is_unsigned_integer` are
refactored to be variable templates instead of class templates.
- the two traits are merged into a single header
`<__type_traits/integer_traits.h>`.
- `__libcpp_signed_integer`, `__libcpp_unsigned_integer` and
`__libcpp_integer` are moved into the same header.
- The above mentioned concepts are renamed to `__signed_integer`,
`__unsigned_integer` and `__signed_or_unsigned_integer` respectively.
