[utils] Examine need for `FixedBytes` newtype

[clabby]

Overview

commonware-utils has the FixedBytes newtype, which wraps a [u8; N]. We should examine whether or not we need it - it'd be nice to delete it in favor of using [u8; N] directly.

[clabby]

@roberto-bayardo Assigned you to add to your radar; Feel free to drop assignment if someone else should take it on.

[roberto-bayardo]

thx, let's tag @patrick-ogrady in case he had a reason to keep this its own explicit type.