Remove `+ AsMut<[u8]>` from various trait definitions

[DaniPopes]

Describe the feature

Requiring + AsMut<[u8]> is a major footgun, as it means two opposite things in the implementations of BufMut for Vec<u8> and &mut [u8]:

• Vec<u8> returns the written-to, initialized slice of bytes; this is used internally in e.g. TransactionSignedNoHash to fill the buffer with zeros while we still don't have the value we want to write at the start of the buffer (like flags for lengths)
• &mut [u8] returns the uninitialized spare capacity, which is not what's expected as seen above

I tried using this API for https://github.com/paradigmxyz/reth/issues/11296 but it fails because sometimes we use &mut [u8] as the BufMut which makes logic for the Vec<u8> usage incorrect.

We should remove the BufMut + AsMut pair of traits and write our own wrapper trait for BufMut that is implemented on Vec, SmallVec, ArrayVec, and slices that allows for indexing into the already-written-to buffer. This allows performance improvements like https://github.com/paradigmxyz/reth/issues/11296 by making it possible to go back and fill a value that was zero'd because the value is only available after encoding the entire struct

Additional context

No response