Feature request: in `Base.stale_cachefile`, for determining cachefile freshness, use a hash of the source file contents (instead of comparing mtimes)

[DilumAluthge]

Currently, in the Base.stale_cachefile function, in order to determine whether or not a .ji cachefile is fresh relative to its .jl source files, for each source file, we compare ftime_req (the original mtime of the source file as recorded in the header of the cachefile) to ftime (the current mtime of the source file).

The advantage of this approach is that it is fast. The disadvantage is that we have to do all kinds of workarounds to deal with mtime issues in different situations. For example:

julia/base/loading.jl

Lines 2052 to 2054 in 0a55a8e

	# Issue #13606: compensate for Docker images rounding mtimes
	# Issue #20837: compensate for GlusterFS truncating mtimes to microseconds
	# The `ftime != 1.0` condition below provides compatibility with Nix mtime.

Instead of using mtime, it would be nice if we could compute a hash of the contents of the source files; then, we invalidate the cachefile if and only if the hash has changed.

The disadvantage of using hashes is that it would be slower than using mtimes. The advantage is that we can get rid of all of the workarounds that are necessary for the mtime approach. We can also ensure that we only invalidate a cachefile if the source files have actually had their contents modified.

[IanButterworth]

If it moves to using hashes, perhaps first do a file size check to allow fail-fast?

[Keno]

Also consider making the hashing compatible with fs-verity, so if that's enabled on a file, you can just pick up the hash from there without having to read it.