Optimize byte validity representation

[jswrenn]

In rust-lang/rust#139990, we had to reduce the size of the types under test in tests/ui/transmutability/maybeuninit.rs to accommodate a performance regression that stems from the unwieldy representation of u8 as a DFA with 255 edges. Consequently, the DFA of ExplicitlyPadded, in that UI test (which is only four bytes wide), is formidable:

It might be possible to instead represent edges as bit sets, thereby condensing the above graph into just a few edges. Once this optimization is made, the tests in tests/ui/transmutability/maybeuninit.rs should be restored back to their original size.