[roll] Roll fuchsia [mold] Fixed UB issues.

There were two undefined behavior issues identified by miri:

1. a slice::from_raw_parts call that was using a bigger length
   than allowed for the allocation
2. lack of compliance with Rust's still unstable Stacked Borrows
   in the SplitsCache type because the creation of a secondary
   mutable reference for an allocation whose pointer was still
   going to be used.

Examples:

1. let mut array = [1i32, 2, 3];
   let slice = unsafe { slice::from_raw_parts(array.as_ptr(), 4) };

2. let mut o = ();
   let p = &mut o as *mut _;

   &mut o; // mutably ref'd even though p is a deref'd later

   unsafe { *p = (); }

The fix for the first issue was trivial.

The fix for the second involved refactoring Layout and SplitsCache.
SplitsCache, now renamed into SliceCache, uses a global pointer to store
the latest use in SliceCache::access, while blocking if there is any
other instance trying to use the same pointer.

This is a conservative solution since concurrent access with even
disjoint caches is not supported.

Layout was also simplified by removing the TileWriter type and simply
moving the logic to the Layout trait.

Testing: fx test surpass-tests && fx test mold-tests

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/643829
Original-Revision: cc2ad85e51fa05659d316c1b61a1d71f215d42f6
GitOrigin-RevId: 1adcbe3d7166f628d28738ac7be2d7aa0a7381e3
Change-Id: I4ec94f47d82053b69eac6834781db1b5370e80b6