Add a basic Core.Print function for ints. (#4078)

We'd been discussing that explorer remains necessary for print, and I
was wondering if this kind of approach would be okay (we _probably_ want
this to work, based on #2110, albeit with more overloads -- but I don't
think there's a good way to support overloads at the moment).

```
╚╡../bazel-bin/examples/sieve
2
3
5
7
11
13
17
19
23
29
31
37
41
43
...
```

examples/sieve.carbon

@@ -116,6 +116,7 @@ fn Run() -> i32 {
   while (n < 1000) {
     if (s.is_prime[n]) {
       ++number_of_primes;
+      Core.Print(n);
       s.MarkMultiplesNotPrime(n);
     }
     ++n;