Bump `most-positive-fixnum`?

[fosskers]

Hi there, thanks for ABCL as always. While profiling, I noticed that ABCL differs from its peers in terms of the size of its most-positive-fixnum:

| Compiler | most-positive-fixnum |
|----------+----------------------|
| SBCL     |  4611686018427387903 |
| ECL      |  2305843009213693951 |
| Clasp    |  2305843009213693951 |
| CCL      |  1152921504606846975 |
| Allegro  |  1152921504606846975 |
| Clisp    |      281474976710655 |
| ABCL     |           2147483647 |

Is there a possibility of this being bumped?

[easye]

A reasonable request. I need to add some tests to our possible size, and will bump our limit to something contemporary.

[fosskers]

Thank you!