Extract #system_read and #system_write for FileDescriptor and Socket

[straight-shoota]

Currently, individual system implementations of System::FileDescriptor and System::Socket implement IO#unbuffered_read and IO#unbuffered_write directly. This patch pulls the implementation up to be shared between all platforms, and instead delegates to #system_read and #system_write.

#unbuffered_write is now responsible for ensuring that the entire content of the slice is written, calling system_write multiple times if necessary. (Looks like this wasn't implemented correctly in Socket on Windows before)

This is a simplification step in preparation for a bigger refactoring which will delegate #system_read and #system_write to the event loop (crystal-lang/rfcs#7).