PERL-369 fix segfault on pure 32bit perl

Segfault was due to 64 bit deserialization from BSON when Perl doesn't
support 64-bit integers (even if the compiler does).

Fix came in two parts:

(1) on pure 32-bit perl, use Math::BigInt objects for 64bit types
(2) ensure that cursor IDs can be coerced from Math::BigInt objects