Create an untyped API that works on [u8] slices

[elBoberido]

Brief feature description

Create an untyped API that works on [u8] slices.

Detailed information

Add allocates method to the publisher impl like

pub fn allocate(length: usize) -> SampleMut<[u8]> {}

pub fn allocate_aligned(length: usize, alignment: usize) -> SampleMut<[u8]> {}

The subscriber shall also be able to work with slices.

This should enable us to expose &mut [u8] and have a nice compatibility with the bytes::BufMut trait from the bytes crate

[elBoberido]

@neilisaac this will not be part of v0.1 but I plan to have this as soon as possible in v0.2