Fallible operations in zerovec

[Manishearth]

Part of https://github.com/unicode-org/icu4x/issues/1082

The zerovec crate has a bunch of operations which may panic, specifically most of the mutation operations as well as VarZeroVec construction operations where the final slice may be larger than a u32 in length.

We should have fallible APIs; perhaps making them the default.

[Manishearth]

This would be obviated by https://github.com/unicode-org/icu4x/issues/1443 which is likely to happen. Keeping the issue open anyway until that is fixed