Add benchmarks for signing with null and non-null merkle_root arguments.
Null and non-null merkle_root arguments will apply the taptweaks
H_TapTweak(P) and H_TapTweak(P | merkle_root), respectively, to the
private key during signing.
This benchmark is added to verify there are no significant performance
changes after moving the taptweak signing logic in a later commit.
Co-authored-by: l0rinc <pap.lorinc@gmail.com>