This is hopefully going to make navigating the source tree easier.
Hopefully.
The only types that don't get their own files are:
* function types (UserFunction, BuiltinFunction, Method), which all live
in obj/function.rs
* Nil, which lives in obj.rs
* Obj, which lives in obj.rs
Type definitions and init_types now live in obj/ty.rs.
New obj::prelude module for common imports.
Signed-off-by: Alek Ratzloff <alekratz@gmail.com>