parser: check (mut f Foo) syntax

vlib/math/big/big.v

@@ -124,10 +124,10 @@ pub fn cmp(a Number, b Number) int {
 pub fn (a Number) is_zero() bool {
 	return C.bignum_is_zero(&a) != 0
 }
-pub fn (a mut Number) inc() {
+pub fn (mut a Number) inc() {
 	C.bignum_inc(a)
 }
-pub fn (a mut Number) dec() {
+pub fn (mut a Number) dec() {
 	C.bignum_dec(a)
 }
 pub fn pow(a Number, b Number) Number {