all: change f mut Foo to mut f Foo

vlib/builtin/array_test.v

@@ -311,7 +311,7 @@ fn test_fixed() {
 	assert nums2[c_n - 1] == 0
 }
 
-fn modify(numbers mut []int) {
+fn modify(mut numbers []int) {
 	numbers[0] = 777
 }
 
@@ -328,13 +328,13 @@ fn test_mut_slice() {
 	*/
 }
 
-fn double_up(a mut []int) {
+fn double_up(mut a []int) {
 	for i := 0; i < a.len; i++ {
 		a[i] = a[i]*2
 	}
 }
 
-fn double_up_v2(a mut []int) {
+fn double_up_v2(mut a []int) {
 	for i, _ in a {
 		a[i] = a[i]*2 // or val*2, doesn't matter
 	}

vlib/builtin/map_test.v

@@ -160,7 +160,7 @@ fn test_string_arr() {
 	assert m['a'][1] == 'two'
 }
 
-fn mut_map(m mut map[string]int) {
+fn mut_map(mut m map[string]int) {
 	m['a'] = 10
 }
 

vlib/builtin/sorted_map.v

@@ -112,7 +112,7 @@ fn (mut m SortedMap) set(key string, value voidptr) {
 	}
 }
 
-fn (mut n mapnode) split_child(child_index int, y mut mapnode) {
+fn (mut n mapnode) split_child(child_index int, mut y mapnode) {
 	mut z := new_node()
 	z.size = mid_index
 	y.size = mid_index
@@ -353,7 +353,7 @@ pub fn (mut m SortedMap) delete(key string) {
 
 // Insert all keys of the subtree into array `keys`
 // starting at `at`. Keys are inserted in order.
-fn (n &mapnode) subkeys(keys mut []string, at int) int {
+fn (n &mapnode) subkeys(mut keys []string, at int) int {
 	mut position := at
 	if !isnil(n.children) {
 		// Traverse children and insert