An interesting byproduct was finding a problematic `assert` in the
Swift headers. An incomplete `FallthroughStmt` was asserting on having
a destination. I did not find any other sensible way of getting rid of
the crash when running in debug mode than to patch the header.
This replaces usages of `llvm::fs` and string manipulation with
`std::filesystem`, also replacing `std::string` with
`std::filesystem::path` where it made sense.
Moreover MD5 hashing used in macOS file remapping was replaced by
SHA256 hashing using a small header-only SHA256 C++ library with an
MIT license, https://github.com/okdshin/PicoSHA2.
File contents hashing was relocated to the newly created `file` library
for later planned reuse.
