Use std::chrono::high_resolution_clock for LV::Time::now()

LV::Time::now() currently uses platform-specific implementations for its high resolution clock. Port it to use C++11's std::chrono::high_resolution_clock instead. It has a nicer interface and has better portability.

Caveat: VS2012's implementation does not use QueryPerformanceCounter and has resolution issues (
source). Resolved in VS2015. We currently only target MinGW and now require C++20, so it does not really matter.