compiler: add [..2] & [2..] support for slices

vlib/builtin/array_test.v

@@ -118,10 +118,13 @@ fn test_right() {
 	a := [1, 2, 3, 4]
 	b := a.right(1)
 	c := a[1..a.len]
+	d := a[1..]
 	assert b[0] == 2
 	assert b[1] == 3
 	assert c[0] == 2
 	assert c[1] == 3
+	assert d[0] == 2
+	assert d[1] == 3
 }
 
 fn test_right_with_n_bigger_than_array_size() {
@@ -142,10 +145,13 @@ fn test_left() {
 	a := [1, 2, 3]
 	b := a.left(2)
 	c := a[0..2]
+	d := a[..2]
 	assert b[0] == 1
 	assert b[1] == 2
 	assert c[0] == 1
 	assert c[1] == 2
+	assert d[0] == 1
+	assert d[1] == 2
 }
 
 fn test_slice() {