#1 Switch to std::chrono for timing.

[Patrick M. Niedzielski]

This is has two parts:

• Remove hummstrumm::engine::date code (functionality is in standard library, for the most part).
• Take out the high resolution timing code from platform-specific subsystem. std::chrono::high_resolution_clock gives us the same thing, only cross-platform.

[Patrick M. Niedzielski]

Fixed on unstable with commit af1ffb6396aa23718d1b20ab9ce364aaffdb140a.