array: add `array.trim()`

vlib/builtin/array.v

@@ -152,6 +152,14 @@ pub fn (a mut array) clear() {
 	a.len = 0
 }
 
+// trims the array length to "index" without modifying the allocated data. If "index" is greater
+// than len nothing will be changed
+pub fn (a mut array) trim(index int) {
+	if index < a.len {
+		a.len = index
+	}
+}
+
 // Private function. Used to implement array[] operator
 fn (a array) get(i int) voidptr {
 	$if !no_bounds_checking? {

vlib/builtin/array_test.v

@@ -592,3 +592,20 @@ fn test_clear() {
 	arr.clear()
 	assert arr.len == 0
 }
+
+fn test_trim() {
+	mut arr := [1,2,3,4,5,6,7,8,9]
+	assert arr.len == 9
+	
+	arr.trim(9)
+	assert arr.len == 9
+	assert arr.last() == 9
+	
+	arr.trim(7)
+	assert arr.len == 7
+	assert arr.last() == 7
+	
+	arr.trim(2)
+	assert arr.len == 2
+	assert arr.last() == 2
+}