Cleanly separate `TapTree` and `NodeInfo`. Fix serde not respecting
invariants for several data structures
Repurpose some tests from removed taproot builder for taptree
"schnorr" is a dirty word; the current `schnorr` module defines a
`Signature` that includes a sighash type, this sighash type is a bitcoin
specific construct related to taproot. Therefore the `Signature` is
better named `taproot::Signature`. Note also that the usage of `schnorr`
in `secp256k1` is probably justified because the
`secp256::schnorr::Signature` is just doing the crypto.
While we are at it, update docs and error messages to use "taproot"
instead of "schnorr". Also change function names and identifiers that
use "schnorr".
Tobin C. Harding
sanket1729
DanGould
Arturo Marquez