Support for Schnorr signatures and integration in SignatureCheckers (BIP 340)

This enables the schnorrsig module in libsecp256k1, adds the relevant types
and functions to src/pubkey, as well as in higher-level `SignatureChecker`
classes. The (verification side of the) BIP340 test vectors is also added.

src/script/script_error.h

@@ -66,6 +66,11 @@ typedef enum ScriptError_t
     SCRIPT_ERR_WITNESS_UNEXPECTED,
     SCRIPT_ERR_WITNESS_PUBKEYTYPE,
 
+    /* Taproot */
+    SCRIPT_ERR_SCHNORR_SIG_SIZE,
+    SCRIPT_ERR_SCHNORR_SIG_HASHTYPE,
+    SCRIPT_ERR_SCHNORR_SIG,
+
     /* Constant scriptCode */
     SCRIPT_ERR_OP_CODESEPARATOR,
     SCRIPT_ERR_SIG_FINDANDDELETE,