[refactor] add alias for Schnorr signatures and remove size magic number

Summary:
Schnorr signatures repeat as std::array<uint8_t, 64> throughout the
code. this defines them in a single alias and replaces the magic number
with CPubKey::SCHNORR_SIZE

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9251