Prepare and_then/map/transform/map_error/or_else for SFINAE-friendliness

[Quuxplusone]

This incidentally simplifies the preprocessor stuff, by making these codepaths look the same in both C++11 and C++14. The old code had the weird effect that the C++11 codepath was slightly more SFINAE-friendly than the C++14 codepath.

This patch doesn't actually make and_then/map/transform/map_error/or_else be SFINAE-friendly. But it lays the groundwork for adding further constraints to the enable_if stuff in and_then_impl etc., which could make these functions fully SFINAE-friendly without needing to touch the non-_impl versions again.