Add custom error type for OpenSSL bindings

[mbestavros]

This PR adds KeylimeCryptoError, a custom error type, to be used by all bindings in crypto.rs. This allows a uniform return type that handles a variety of different errors thrown by implementations of the crypto wrappers, without complex logic or loss of error information. More information can be found at the corresponding issue, #42.

Notes:

• KeylimeCryptoError is not included as a return type for existing functions at the moment. I intend to update the existing bindings with the new error type in a separate PR.

• Currently, KeylimeCryptoError preserves OpenSSL ErrorStack structs in their entirety by simply wrapping them. This allows further reference down the line if needed, though suggestions are welcome for simplification.

• Right now, KeylimeCryptoError can handle std::io::Error and openssl::error::ErrorStack types; those two are the only ones that crypto.rs requires at the moment. Should new crypto bindings require additional error handling, these can be expanded accordingly.

Tagging @frozencemetery, @leonjia0112, @lukehinds for review.