First stage of struct variant field visibility changes

We need a snapshot before the parser can be adjusted.
Make Int inherit from Ord.

Previously Int inherited from PartialOrd (via Primitive)
but not Ord.  But integers have a total order, so
inheriting from Ord is appropriate. Fixes #18776.
Fix upvars sometimes not being marked as used mutably

Drill down the loan path for McDeclared references as well since
it might lead to an upvar.  Closes #18769
Update test to cover #18769
Implement collection views API for TrieMap.
Attempt to fix the problem with failing distcheck.
mk: Fix configuration of version commit information

Commit bec2ee77f78b4bb8a503101091272a634a273a1c started quoting paths
discovered as part of the `probe` function, which includes git.  The
`make` `wildcard` function appears to be incompatible with quoted
paths so this check in the makefile now fails. Employing `wildcard`
here appears to only re-verify that git actually exists, which the
configure script already did, so I've just removed it.

Additionally, with the quoted paths the `subst` function should no
longer be needed, so I've removed it as well.

Closes #18771
Rename deprecated EnumSet methods in unit tests.
syntax: improve message for misused inner attributes
Make os::getcwd() return IoResult<Path>

os::getcwd() panics if the current directory is not available. According
to getcwd(3), there are three cases:

- EACCES: Permission denied.
- ENOENT: The current working directory has been removed.
- ERANGE: The buffer size is less than the actual absolute path.

This commit makes os::getcwd() return IoResult<Path>, not just Path,
preventing it from panicking.

As os::make_absolute() depends on os::getcwd(), it is also modified to
return IoResult<Path>.

[breaking-change]
Make os::change_dir() return IoResult<()>

os::change_dir() returns bool, without a meaningful error message.
Change it to return IoResult<()> to indicate what IoError caused the
failure.

[breaking-change]
Make os::setenv() and os::unsetenv() return IoResult<()>

Those functions can fail if:

- EINVAL: The name is empty, or contains an '=' character
- ENOMEM: Insufficient memory