C++2a-based likely/unlikely/predict... macros#29

I just pushed some implementations of `HEDLEY_LIKELY`, `HEDLEY_UNLIKELY`, `HEDLEY_PREDICT`, `HEDLEY_PREDICT_TRUE`, and `HEDLEY_PREDICT_FALSE` to a new wip/c++2a branch (commit ID f57289ea9774401c1f872bacd1067564b1011226). They work as expected on GCC 9 (AFAIK the only compiler which supports the `[[likely]]`/`[[unlikely]]` attributes, but I'd like to do a bit more testing before including them in a release. The good news is that Microsoft generally implements new C++ features (unlike C), so the next release of Visual Studio will probably support the required attributes, meaning we may actually get a working implementation on MSVC. Most other compilers already support `__builtin_expect`, so MSVC is where this should be important.