add range constraint type

examples/programs/for_loop/main.bro

@@ -1,6 +1,6 @@
 # Milestone 5: ranges and sequence for loops.
 
-pass :: func(value int) int {
+pass :: func(value range) range {
 	return value
 }
 

examples/programs/for_loop_edges/main.bro

@@ -1,4 +1,4 @@
-make_range :: func(calls @mut i32, end usize) int {
+make_range :: func(calls @mut i32, end usize) range {
 	calls^ = calls^ + 1
 	return 0..end
 }