builtin: add an array.drop(n) method (#13907)

vlib/builtin/array_test.v

@@ -1046,6 +1046,40 @@ fn test_trim() {
 	assert arr.last() == 2
 }
 
+[manualfree]
+fn test_drop() {
+	mut a := [1, 2]
+	a << 3 // pushing assures reallocation; a.cap now should be bigger:
+	assert a.cap > 3
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+
+	a.drop(-1000)
+	assert a == [1, 2, 3] // a.drop( negative ) should NOT modify the array
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+
+	a.drop(2)
+	assert a == [3]
+	assert a.cap > a.len
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+
+	a.drop(10)
+	assert a == []
+	assert a.cap > a.len
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+
+	a << 123
+	a << 456
+	a << 789
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+	assert a == [123, 456, 789]
+
+	a.drop(10)
+	assert a == []
+	// eprintln('>>> a.cap: $a.cap | a.len: $a.len')
+
+	unsafe { a.free() } // test offset OK
+}
+
 fn test_hex() {
 	// array hex
 	st := [byte(`V`), `L`, `A`, `N`, `G`]