Export sslHandle in net and asyncnet

[ardek66]

Summary

Export the sslHandle field of sockets in net and asyncnet respectively,
just like SslContext exports context.

Description

Currently, the only way of using libssl procedures that take an ssl handle as a parameter(be them from openssl or imported through FFI separately) is by either rewritting the socket wrapping procedure or include-ing net or asyncnet, both of which being quite overkill for achieving simple tasks, even calling simple procedures from the stdlib openssl such as SSL_get_error or SSL_get_peer_certificate.

The SslContext itself allows us to access the low-level context, why shouldn't Socket?

Alternatives

One could add the needed procedures to the nim openssl, but then comes the problem if it is actually needed, or if it keeps
compatibility with the rest, making development of both openssl and the program itself harder.

Additional Information

This problem can be seen discussed here

[ringabout]

@ardek66

make a PR?