WIP: Add (picrin socket) and (srfi 106).

**Do not merge this PR at this moment.**

I need socket support in picrin, so I began to write a binding. However, I'm not sure that my usage of API and coding style is correct. Thus, I'd make a RFC PR with a part of the binding. Would you review me before I write rest of the socket binding? Any suggestions are welcome.

Also, let me know if anyone already wrote and published a socket binding.

Known problems:
* Incomplete implementation of SRFI 106
* Incomplete tests
* Lack of Windows (Winsock2) support
* Lack of platform checking in CMakeLists.txt