Fixes #384.

A set is defined by the same semantics that the String#count
method in Ruby defines.

Removed an IO specs (removed one that wasn't portable, actually) and some refactors. Added IO#to_fd_io to convert an IO to a FileDescriptorIO if possible

Also port Process.run to using it

When issuing a "can't send closure to C function" show the closured variables if the compiler knows them

Make C structs and unions inherits from Struct. Make it work so you can invoke `to_s` on them (for unions it just prints the name of the type).

Added spec for GithubDependency#target_dir (and move its initialization to the initialize so its not nilable)

target_dir was not being called, resulting in strange behaviour.

[Docs] Support language in code section ("text" or "crystal" right now) to disable syntax highlighting

To be logical with Addr#ip_address