rlp: reduce allocations for big.Int and byte array encoding (#21291)

This change further improves the performance of RLP encoding by removing
allocations for big.Int and [...]byte types. I have added a new benchmark
that measures RLP encoding of types.Block to verify that performance is
improved.

rlp/typecache.go

@@ -210,6 +210,10 @@ func isUint(k reflect.Kind) bool {
 	return k >= reflect.Uint && k <= reflect.Uintptr
 }
 
+func isByte(typ reflect.Type) bool {
+	return typ.Kind() == reflect.Uint8 && !typ.Implements(encoderInterface)
+}
+
 func isByteArray(typ reflect.Type) bool {
 	return (typ.Kind() == reflect.Slice || typ.Kind() == reflect.Array) && isByte(typ.Elem())
 }