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/test/fuzz/signature_checker.cpp

@@ -33,6 +33,11 @@ public:
         return m_fuzzed_data_provider.ConsumeBool();
     }
 
+    bool CheckSchnorrSignature(Span<const unsigned char> sig, Span<const unsigned char> pubkey, SigVersion sigversion, ScriptError* serror = nullptr) const override
+    {
+        return m_fuzzed_data_provider.ConsumeBool();
+    }
+
     bool CheckLockTime(const CScriptNum& nLockTime) const override
     {
         return m_fuzzed_data_provider.ConsumeBool();