KJ Filesystem API

[kentonv]

I've long wanted a filesystem API that:

• Is resistant to path injection bugs.
• Uses a representation of paths that is easy to manipulate (flat strings with / as separators are actually incredibly tedious!).
• Canonicalizes ".." logically (by removing the previous path component) rather than physically (by climbing the directory tree), especially when dealing with the current working directory.
• Is RAII.
• Does not assume a singleton filesystem (= implements abstract interfaces so that you can provide a different implementation when desired).
• Throws exceptions on error, but lets you opt-in to programatically handling the kinds of errors that one commonly wants to handle (namely, doesn't-exist and already-exists).
• Directly supports the write-to-temp-then-atomically-replace pattern.
• Directly supports the common operations "recursively delete directory" and "automatically create parent directories".
• Directly supports a copy operation, with smart semantics around sparse files.
• Makes mmap easy.
• Automatically opens all file descriptors close-on-exec.
• Utilizes Linux features like renameat2(), FICLONE (copy-on-write files), O_TMPFILE, etc. when available, but falls back to generic POSIX when not.
• Much more...

So this is my attempt.

[kentonv]

(I just merged everything except the filesystem API itself. Github seems very confused about this...)

[kentonv]

Gonna merge this because I'm using it in a project, although currently only Ekam will build it. I guess we need a Win32 version before we can make it part of the main builds.

I already cut the 0.6 release branch so this will land in 0.7.