Fix 8.4 incompatibilities

[Firehed]

8.4 starts issuing warnings about implicit nullable parameters, e.g. int $foo = null should be ?int $foo = null. This change corrects the signatures for compatibility.

This also relatedly blocks versions of PHP with a regression in GMP, demonstrated at https://3v4l.org/lnhXp

[sop]

Thanks for the PR!
While it's true that those PHP versions you added as conflicting seem to cause errors, i think it's fairly safe to allow distributing regardless. So i removed the restrictions from composer.json.

[Firehed]

Glad to help!

FWIW, the following patch avoids that regression:

diff --git a/lib/ASN1/Util/BigInt.php b/lib/ASN1/Util/BigInt.php
index 28e5ccd..7d25660 100644
--- a/lib/ASN1/Util/BigInt.php
+++ b/lib/ASN1/Util/BigInt.php
@@ -183,7 +183,10 @@ class BigInt
             } while ($tmp > 128);
         }
         // compute two's complement 2^n - x
-        $num = gmp_pow('2', 8 * $width) - $num;
+        $base = gmp_init('2');
+        $exponent = 8 * $width;
+        $num = $base ** $exponent - $num;
         $bin = gmp_export($num, 1, GMP_MSW_FIRST | GMP_BIG_ENDIAN);
         // if first bit is 0, prepend full inverted byte to represent negative two's complement
         if (!(ord($bin[0]) & 0x80)) {

I believe this will work all the way back to 7.2 (GMP operator overloads were added back in 5.6), but I didn't see CI results so I left it out for now in case other areas are similarly affected. Firehed/webauthn-php#89 and php/php-src#16870 have more context if you're interested.