The type returned by `Witness::taproot_control_block()` was just `&[u8]`
which wasn't very nice since users then had to manually decode it which
so far also required allocation. Thanks to previous improvements to
`ControlBlock` it is now possible to return a `ControlBlock` type
directly.
To avoid expensive checks, this change adds a new type
`SerializedXOnlyPublicKey` which is a wrapper around `[u8; 32]` that is
used in `ControlBlock` if complete checking is undesirable. It is then
used in the `ControlBlock` returned from
`Witness::taproot_control_block`. Users can still conveniently validate
the key using `to_validated` method.
It then uses this type in the recently-added `P2TrSpend` type. As a side
effect this checks more properties of `Witness` when calling unrelated
methods on `Witness`. From correctness perspective this should be OK: a
witness obtained from a verified source will be correct anyway and, if
these checks were done by the caller, they can be removed.
From performance perspective, if the `Witness` was obtained from a
verified source (e.g. using Bitcoin Core RPC) these checks are wasted
CPU time. But they shouldn't be too expensive, we already avoid
`secp256k1` overhead and, given that they always succeed in such case,
they should be easy to branch-predict.
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