Implement Taproot validation (BIP 341)

This includes key path spending and script path spending, but not the
Tapscript execution implementation (leaf 0xc0 remains unemcumbered in
this commit).

Includes constants for various aspects of the consensus rules suggested
by Jeremy Rubin.

src/script/script_error.h

@@ -56,6 +56,7 @@ typedef enum ScriptError_t
     /* softfork safeness */
     SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS,
     SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM,
+    SCRIPT_ERR_DISCOURAGE_UPGRADABLE_TAPROOT_VERSION,
 
     /* segregated witness */
     SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH,
@@ -70,6 +71,7 @@ typedef enum ScriptError_t
     SCRIPT_ERR_SCHNORR_SIG_SIZE,
     SCRIPT_ERR_SCHNORR_SIG_HASHTYPE,
     SCRIPT_ERR_SCHNORR_SIG,
+    SCRIPT_ERR_TAPROOT_WRONG_CONTROL_SIZE,
 
     /* Constant scriptCode */
     SCRIPT_ERR_OP_CODESEPARATOR,