Use fix-sized primops for fixed size boxed types

Progress towards #19026 (closed), fixes #19931 (closed)

Now that we have the 32-bit primpos (from !4698 (closed) which is contained), we can do this for all over the 8-, 16-, and 32-bit boxed types. This will shrink and declutter the core.

Unfortunately I've been getting a GHC panic. But it went away with Devel2, so I'm back to fixing obvious issues.

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