Move away from mod.rs

[Profpatsch]

According to the description in the current rust book and the issues tracked by rust-lang/rust#53125, the preferred way of doing nested modules in newer rust versions is the directory/filename combination.

So instead of

foo/bar/mod.rs
foo/bar/abc.rs

where you have a module foo::bar (implemented in foo/bar/mod.rs) and a module foo::bar::abc (implemented in foo/bar/abc.rs), you would do

foo/bar.rs
foo/bar/abc.rs

where foo:bar is implemented in in foo/bar.rs.