load[_unchecked] and store[_unchecked] take an unnecessary offset parameter

[eira-fransham]

Currently load, load_unchecked, store and store_unchecked essentially reimplement slicing logic with their offset parameter. They could instead take only one parameter (the slice) and have the user replace:

u32s::load(my_slice, n)

with

u32s::load(&my_slice[n..])

I believe there is no downside to doing this other than the fact that it would require a major version bump (although every version bump is major right now).

[gnzlbg]

I ran into this while reworking the portable vector types API and decided to drop the offset parameter.

[gnzlbg]

This was fixed in #338 .