Converts all use of boost::filesystem to std::filesystem.
For macos and potentially other exotic systems where std::filesystem
isn't available, we use ghc::filesystem instead (which is a drop-in
replacement for std::filesystem, unlike boost::filesystem).
This also greatly changes how we handle filenames internally by holding
them in filesystem::path objects as soon as possible (using
fs::u8path()), rather than strings, which avoids a ton of issues around
unicode filenames. As a result this lets us drop the boost::locale
dependency on Windows along with a bunch of messy Windows ifdef code,
and avoids the need for doing gross boost locale codecvt calls.
common/util.h has become something of a dumping ground of random
functions. This splits them up a little by moving the filesystem bits
to common/file.h, the sha256sum functions to common/sha256sum.h, and the
(singleton) signal handler to common/signal_handler.h.
