Use boost.interprocess's mmap wrapper in the PCM provider
Extract the rest of the mmap logic from the PCM provider. Closes #934.
Use read_file_mapping directly in the HD audio provider
Use mmap for reading subtitles from Matroska files

Cuts cold-cache read time for an arbitrary 1 GB file read over a network
off a USB 2.0 hard drive from ~70 seconds to ~45 seconds.
Fix crash_writer_minidump.cpp's newlines
Simplify PCMAudioProvider a bit and pull it fully into the cpp file
Remove unused variable