Add modules to ruby-sys

[d-unsed]

Add basic support for Modules.

[danielpclark]

Can this be prioritized? I would greatly appreciate it.

Class already has:

• rb_define_module
• rb_define_module_under

I see we need:

• rb_define_module_function
• rb_define_private_method

Maybe rb_define_alloc_func see Helix line: libcruby-sys/src/lib.rs#L199