Added more tests, function calls work

Fixed a bunch of bugs in other opcodes, too

test/isqrt.c0

@@ -0,0 +1,10 @@
+int main () {
+  int n = 15122;
+  int i = 0; int k = 0;
+  while (k <= n) {
+    k += 2*i + 1;
+    i++;
+  }
+  return i-1;
+}
+