Simplify and fix some correctness issues with fixed-size byval arrays

[thomcc]

• Simplifies the code a good amount, mostly reling on postgres's documentation about byval types to be correct (and for the optimizer see through u32 -> Datum -> u32 and such).
• Inlines some datum functions which are required for the optimizer to actually see through that in all cases.
• Debug asserts about alignment more consistently, but it shouldn't ever be hit on x86_64 or aarch64.
• if target_pointer_width == "32" some of the slice impls were busted, so they've been disabled on 32 bit. (Note that 32 bit support is quite experimental, and there are probably more things we do wrong there)

[eeeebbbbrrrr]

This is v nice, @thomcc. Great cleanup and simplification and correctness. Much appreciated! Feel free to merge when you're ready.