avoid unnecessary #to_io calls

avoid unnecessary #to_io calls

Calling `#to_io' is only necessary when we're handling an argument
from user code where the user could pass a non-IO object.
`#to_io' calls are a waste of time when we create the IO object
ourselves (in `Raindrops::InetDiagSock.new').

This allows us to define the `my_fileno' macro for Ruby 3.1+
users to call the new `rb_io_descriptor' function directly
without an extra C stack frame.  This also allows us to get
rid of nesting CPP directives inside C functions which (IMHO)
improves readability.

Furthermore, any necessary #to_io calls using `rb_convert_type'
can be replaced with `rb_io_get_io' to decrease code size.
`rb_io_get_io' has been in ruby/io.h since Ruby 1.9.2 and
there's no expectation that it'd be deprecated since it only
deals with opaque `VALUE' types.