Problem:
- It is useful to have a macro that wraps a potentially-`constexpr`-usable value
  in `stdx::ct` when possible, to preserve its `constexpr` properties.

Solution:
- Add `CT_WRAP` that does this.

Notes:
- See intel/compile-time-init-build#743 for some parts
  of this in CIB; this part probably belongs in stdx.

Problem:
- It's useful to have `map`/`transform` in the preprocessor.

Solution:
- Add `pp_map.hpp`.

Problem:
- We want to not forget to apply `CX_VALUE` to arguments in macros when they are
  usable at compile time.

Solution:
- Add `CX_WRAP` which wrap its argument in a `CX_VALUE` when it's possible to
  do so.

Problem:
- It's easy to call `stdx::ct_format` but forget to preserve the constexpr
  nature of the arguments. Especially with literals, e.g.
  `stdx::ct_format<"{}">(42)` results in a runtime format argument.

Solution:
- Add `STDX_CT_FORMAT` which automatically wraps the arguments so that their
  constexpr nature is preserved where possible. `STDX_CT_FORMAT("{}", 42)` is
  now a compile-time formatting operation.