`TaprootMerkleBranchBuf` previously derefed to a slice which lost the
information about length being valid. This commit changes the type
which, while API-breaking, is not disruptive because the type has API
very similar to slice.
`TaprootMerkleBranchBuf` being a vec introduced intermediate allocation
when creating or decoding `Witness`. However the representation on the
wire is the same as in-memory (aside from `#[repr(transparent)]`) so
this allocation wasn't really needed.
This commit introduces `TaprootMerkleBranch` type which is unsized and
can be used in place of `TaprootMerkleBranchBuf` within `ControlBlock`.
Aside from removing the intermediate allocation, this improves the API a
bit: the conversion from array to other type is no longer needed because
it's performed by `ControlBlock` in its methods. Thus, consumers who
have an array can simply set it as `merkle_branch` field and then encode
the `ControlBlock` into witness. A convenience method is also provided
to push the `ControlBlock` along with other parts at the end of the
`Witness`.
Erick Cestari
Martin Habovstiak