Simplify `Nibbles` type

[MegaRedHand]

Currently, our Nibbles type uses a Vec as underlying storage. Using a fixed-bytes array might simplify code. Now that we're moving to a path-based DB implementation, this will be important since we use paths for node indexing.

Apart from this, some of the methods in Nibbles are used for iterating the trie, which makes the internal nibbles implementation harder to understand or modify (see the path-based PR changes to see an example).

[Peponks9]

would like to contribute to this if possible @MegaRedHand

[MegaRedHand]

This issue is mostly a reminder, sorry. We are currently working close to this, so it might be a while before this can be worked on.

[Peponks9]

all good, will look into other issues