Optimize/eliminate bounds checking

TODO: do better at it by tracking min/max values of integers. The
following straightforward code doesn't have its bounds checks removed:

    for _, n := range slice {
        println(n)
    }

src/runtime/string.go

@@ -13,6 +13,7 @@ type _string struct {
 }
 
 // Return true iff the strings match.
+//go:nobounds
 func stringEqual(x, y string) bool {
 	if len(x) != len(y) {
 		return false