Bytes: add support for serializing by extending a Vec<u8, N>

[sosthene-nitrokey]

I would do this by using a trait implemented for &mut [u8], heapless_bytes_v0_3::Bytes<N>, heapless_bytes_v0_4::Bytes<N>, heapless_v0_7::Vec<u8, N>, heapless_v0_8::Vec<u8, N>

For example this would allow the CTAP apps to serialize directly extending the response buffer.

[sosthene-nitrokey]

There is actually already the Writer trait for that, but it's not implemented for heapless types or exposed in a simple function.

[robin-nitrokey]

Implemented in #13.