txscript: Optimize typeOfScript for witness-script-hash

This concludes the process of converting the typeOfScript function to
use a combination of raw script analysis and the new tokenizer instead
of the far less efficient parsed opcodes.

In particular, it converts the detection of witness script hash scripts
to use raw script analysis and the new tokenizer.

With all of the limbs now useing optimized variants, the following is a
before an after comparison of calling GetScriptClass on a large script:

benchmark                     old ns/op     new ns/op     delta
BenchmarkGetScriptClass-8     61515         15.3          -99.98%

benchmark                     old allocs     new allocs     delta
BenchmarkGetScriptClass-8     1              0              -100.00%

benchmark                     old bytes     new bytes     delta
BenchmarkGetScriptClass-8     311299        0             -100.00%

txscript/standard.go

@@ -441,22 +441,15 @@ func typeOfScript(scriptVersion uint16, script []byte) ScriptClass {
 		return ScriptHashTy
 	case isWitnessPubKeyHashScript(script):
 		return WitnessV0PubKeyHashTy
+	case isWitnessScriptHashScript(script):
+		return WitnessV0ScriptHashTy
 	case isMultisigScript(scriptVersion, script):
 		return MultiSigTy
 	case isNullDataScript(scriptVersion, script):
 		return NullDataTy
-	}
+	default:
-
-	pops, err := parseScript(script)
-	if err != nil {
 		return NonStandardTy
 	}
-
-	if isWitnessScriptHash(pops) {
-		return WitnessV0ScriptHashTy
-	}
-
-	return NonStandardTy
 }
 
 // GetScriptClass returns the class of the script passed.