MAX_PUB_EXPONENT is somewhat small #155

The current MAX_PUB_EXPONENT limit for the rsa crate is 1 << (31 - 1), which makes it fail some tests with large e values. For example the limit in ring is (1u64 << 33) - 1.

I'm not sure why the rsa crate chose this value, but there seems to be no risk in aligning with the ring.

https://github.com/RustCrypto/RSA/blob/master/src/key.rs#L20
https://github.com/briansmith/ring/blob/main/src/rsa/public_exponent.rs#L18