BIP-341: Define c[0] as leaf version plus pubkey parity bit

[david-bakin]

trafficstars

Add explicit definition of c[0] as the leaf version and the public key parity bit.

Prior to this change that interesting fact is only implicit in the code of taproot_tweak_pubkey and taproot_sign_script. Making it explicit will help reader understanding of the control block and the script validation rules.

[luke-jr]

@sipa @jonasnick @ajtowns

[jonasnick]

Looks good, feel free to squash

[murchandamus]

@sipa, @jonasnick, @ajtowns: Am I understanding right that you would like this to be merged?