Optimizations for extending bytes

[robin-nitrokey]

Currently, the heapless_bytes::Bytes functions mostly delegate to heapless::Vec. This means that we need to append new items one by one when extending although we could use more efficient operations for byte slices. Alternatives would be to re-implement these operations through Vec::as_mut_ptr and Vec::set_len, or to not use heapless::Vec at all and implement Bytes from scratch.

Affected operations:

• extend_from_slice: calls push_unchecked for every element
• resize/resize_zero/resize_to_capacity: calls push for every new element

If we would drop heapless::Vec, we could benefit from the (still unstable) MaybeUninit::copy_from_slice and MaybeUninit::fill.