fixed_pipeline_state: Hash and compare the whole structure

Pad FixedPipelineState's size to 384 bytes to be a multiple of 16.

Compare the whole struct with std::memcmp and hash with CityHash. Using
CityHash instead of a naive hash should reduce the number of collisions.
Improve used type traits to ensure this operation is safe.

With these changes the improvements to the hashable pipeline state are:

Optimized structure
Hash:            89 ns
Comparison:     103 ns
Construction*:  164 ns
Struct size:    384 bytes

Original structure
Hash:           148 ns
Equal:          174 ns
Construction*:  281 ns
Size:          1384 bytes

* Attribute state initialization is not measured

These measures are averages taken with std::chrono::high_accuracy_clock
on MSVC shipped on Visual Studio 16.6.0 Preview 2.1.