Investigate the possibility of u16 lengths in VarZeroVec

[Manishearth]

Currently VarZeroVec assumes that individual entries may have lengths that fit up to u32 in size. This is quite reasonable, however this is using 4 bytes per element. It may be more efficient to use a u16 here. rkyv uses u16s but is parametrizable.

It's worth discussing whether VarZeroVec<T> will ever contain Ts that are large enough to need more than a u16s worth of space (64 KB) to store their byte length. Note that VarZeroVecs can be nested.

Independently of this, it's also worth considering if VarZeroVec<T> can be made VarZeroVec<T, N = 2> so we can allow for arbitrary length sizes.

[Manishearth]

Closing in favor of #1410

[sffc]

Subsumed by #1443