Work towards #249

These patches are primarily about moving all the vector types and functions into the vector module, and converting all the free functions into methods on those types.

Many existing functions take a pair of MoveType and MoveUntypedVector, pair them and perform a typed operation. This is always unsafe because it requires the two to agree. After these patches, this pairing is always performed in the unsafe constructors of e.g. TypedMoveBorrowedRustVec, which allows some of the subsequent method calls to be declared safe.

During this process I discovered that slight refactorings could drastically change the number of executed rbpf instructions, sometimes exhausting the cpu budget in the bitvector test specifically. Certainly due to whims of the llvm optimizer. So I tracked the perf changes between commits - indicated in some of the commit messages. I did not attempt to maintain perfect parity with existing instruction counts, just to keep them from ballooning too much.

I think the optimizer has particular difficulty seeing all the way through from the constructor to the destructor of the big TypedMoveBorrowedRustVec enum - when it does it can avoid switching multiple times on it. But that's just speculation. I haven't looked closely. I did put #[inline(always)] on the new function of this type as I saw huge decreases in instruction counts doing so - usually inline(always) is not recommended as the llvm inliner tends to make better decisions than people do, but in this case it looked like too big a win to pass up.

I also added docs to the vector module.

This patch also replaces all uses of &mut AnyValue with *mut AnyValue as the former is not sound: safely writing to a &mut AnyValue could result in the actual type having an invalid representation, so writing to an AnyValue must be unsafe (though no code actually does this). I've updated the docs to indicate this.