[DSC] Simplify file mapping to fix intermittent unrelocated pointers

The `FileAccessorCache`'s LRU eviction could discard a file's mapped
data after slide info had already been applied to it. Future accesses to
the file produced a fresh mapping, but failed to reapply the slide info.
This could result in pointers not correctly being slid, such as in
https://github.com/Vector35/binaryninja-api/issues/7689.

The LRU cache existed to stay within OS file descriptor limits, since
the old `MappedFile` held its fd open for the lifetime of the mapping.
There's no real reason for it to hold the file descriptor open like
this. Closing it after `mmap` is sufficient to avoid the file descriptor
limits.

`MappedFileRegion` replaces the combination of `FileAccessorCache`,
`WeakFileAccessor`, and `MappedFileAccessor`. It closes the fd
immediately after mmap, so all files can stay mapped without consuming
descriptors, making the cache unnecessary.

`MappedFileRegion` is owned directly by the `CacheEntry` for its full
lifetime. Slide info is applied exactly once to each `MappedFileRegion`.

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