Add SSL functionality

[noteflakes]

This PR adds SSL functionality, allowing using UringMachine's I/O API as the OpenSSL BIO method:

• Add an API for setting the custom BIO for an SSLSocket.
• Add APIs for reading and writing to an SSLSocket.

This is an alternative solution for integrating UringMachine with OpenSSL to the PR I submitted for adding support for custom BIOs: ruby/openssl#1000

Example usage:

# starting from a raw fd
sock = IO.for_fd(fd)
ctx = some_ssl_ctx
ssl = OpenSSL::SSL::SSLSocket.new(sock, ctx)
# set custom BIO:
machine.ssl_set_bio(ssl)
ssl.accept # or connect

# from this point on you can either use the normal IO methods:
ssl.read(...)
ssl.write(...)

# or, alternatively:
machine.ssl_read(ssl, ...)
machine.ssl_write(ssl, ...)

Performance

Using the custom BIO provides a small performance advantage, but not something that's earth shattering:

            user     system      total        real
stock   4.666976   3.690069   8.357045 (  8.363628)
UM BIO  4.562475   3.594215   8.156690 (  8.163844)
UM I/O  3.768159   3.611543   7.379702 (  7.384713)