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/sign.h

@@ -11,6 +11,7 @@
 #include <pubkey.h>
 #include <script/interpreter.h>
 #include <script/keyorigin.h>
+#include <span.h>
 #include <streams.h>
 
 class CKey;