RSA is slow

[Snowiiii]

• I've searched existing issues and couldn't find a duplicate.

Describe the bug
The current RSA crate we use for Packet encryption has slow key generation, This is basically the only thing which takes time for pumpkin to start, otherwise we should be at 0ms starting time, I searched some similar crates but only found ring which uses OpenSSL mostly, Which we want to avoid especially in rust, See #43

To Reproduce
Start Pumpkin and benchmark the RSA key generation

Expected behavior
Faster key generation

[Snowiiii]

I made an issue on RSA GitHub, See RustCrypto/RSA#454