Fixes evaluation of large positive bigint in the UPLC machine

Fixes #511.

examples/acceptance_tests/087/lib/tests.ak

@@ -0,0 +1,9 @@
+use aiken/cbor.{diagnostic, serialise}
+
+test cbor_serialise_large_num() {
+  serialise(18446744073709551615) == #"c248ffffffffffffffff"
+}
+
+test cbor_diagnostic_large_num() {
+  diagnostic(18446744073709551615) == @"18446744073709551615"
+}