Change Offset.MakeLattice.to_index to return bytes, not bits

[sim642]

Follow-up to #1676 (comment).
A TODO in LockDomain also anticipated this: (* TODO: is this bits or bytes? *).

Fixes Offset.MakeLattice.to_index to return bytes instead of bits, as per documentation:

analyzer/src/cdomain/value/cdomains/offset_intf.ml

Lines 92 to 96 in 153ce28

	val to_index: ?typ:GoblintCil.typ -> t -> idx
	(** Physical memory offset in bytes of the entire offset.
	Used for {!semantic_equal}.
	@param typ base type. *)

Also extracts a few other similar utility functions for offsets and sizes in bytes, not bits. These now assert divisibility by 8 before doing so. Usually this is fine because points to bitfields are impossible.
However, to_index violates it in some memOutOfBounds bitfield tests because the analysis also refers to things accessed more directly, not just those whose addresses have been taken. To handle that, an interval from floor to ceil division is constructed.

Theoretically, doing it in bits is more precise than after having rounded to bytes, but I don't know if this has practical relevance.
The blind division by 8 from #1676 and others wouldn't have done it correctly either for bitfields spanning multiple bytes.