Using `bson_int32?` on an adequately large number crashes with a RangeError.

```
irb(main):001:0> require 'bson'
=> true
irb(main):002:0> 9237164310713597997.bson_int32?
RangeError: bignum too big to convert into `long'
from (irb):2:in `bson_int32?'
from (irb):2
from /usr/local/rbenv/versions/2.2.3/bin/irb:11:in `<main>'
```

As a stopgap solution, I'm using Strings to store these numbers instead, but it's highly space-inefficient.