Starlark: optimize StarlarkInt.Big comparison to StarlarkInt.Int{32,64}

Perform comparison without conversion of smaller integers to
`BigInteger`.  `StarlarkInt.compareTo` does not allocate now.

For this benchmark:

```
def test():
    x = 17 << 77
    for i in range(10):
        print(i)
        for j in range(10000000):
            x > 1

test()
```

```
A: n=27 mean=4.262 std=0.203 se=0.039 min=4.036 med=4.193
B: n=27 mean=4.113 std=0.172 se=0.033 min=3.859 med=4.049
B/A: 0.965 0.941..0.990 (95% conf)
```

Speed up is about 7% when comparing to an integer outside of
`BigInteger` cached range (-16..16).

Finally, `StarlarkInt.Big` to `StarlarkInt.Big` comparison performance
seems to stay the same (within 95% confidence interval after 100
test iterations).

src/main/java/net/starlark/java/eval/StarlarkInt.java

@@ -436,17 +436,32 @@ public int compareTo(StarlarkInt x) {
 
   /** Returns a value whose signum is equal to x - y. */
   public static int compare(StarlarkInt x, StarlarkInt y) {
-    if (x instanceof Int32 && y instanceof Int32) {
-      return Integer.compare(((Int32) x).v, ((Int32) y).v);
+    long xl = 0, yl = 0;
+    boolean xbig = false, ybig = false;
+    try {
+      xl = x.toLongFast();
+    } catch (Overflow unused) {
+      xbig = true;
     }
-
     try {
-      return Long.compare(x.toLongFast(), y.toLongFast());
+      yl = y.toLongFast();
     } catch (Overflow unused) {
-      /* fall through */
+      ybig = true;
+    }
+
+    // If both arguments are big integers, we invoke BigInteger.compare.
+    // If neither argument is big integer, we compare longs.
+    // If only one argument is big integer, it is bigger than other if positive
+    // and smaller otherwise.
+    if (xbig && ybig) {
+      return ((Big) x).v.compareTo(((Big) y).v);
+    } else if (xbig) {
+      return ((Big) x).v.signum();
+    } else if (ybig) {
+      return -((Big) y).v.signum();
+    } else {
+      return Long.compare(xl, yl);
     }
-
-    return x.toBigInteger().compareTo(y.toBigInteger());
   }
 
   /** Returns x + y. */