Preparatory work for secp256k1#995 (stop treating schnorrsig, extrakeys modules as experimental)
Add SECP256K1_DEPRECATED attribute for marking API parts as deprecated
Use SECP256K1_DEPRECATED for existing deprecated API functions
schnorrsig: Rename schnorrsig_sign to schnorsig_sign32 and deprecate
schnorrsig: Adapt example to new API
docs: Fix return value for functions that don't have invalid inputs
_tagged_sha256 simply cannot have invalid inputs.
The other functions could in some sense have invalid inputs but only in
violation of the type system. For example, a pubkey could be invalid but
invalid objects of type secp256k1_pubkey either can't be obtained
via the API or will be caught by an ARG_CHECK when calling pubkey_load.This is consistent with similar functions in the public API, e.g.,
_ec_pubkey_negate or _ec_pubkey_serialize.
This is a backport of secp256k1#1089