Migrate `mod.rs` files to named module files

[LastExceed]

Reasons to prefer named module files:

• mod.rs breaks the otherwise consistent relation between file hierachy and module hierachy
• name and location of a file should be independent of whether or not the module it represents has submodules
• having many files with the same name can be confusing to navigate
• the rust reference recommends using the newer convention

Reasons to stick with mod.rs:

• No transition effort required

[roderickvd]

Fine for me, given it's the latest Rust idiom.

[Marco-Farruggio]

I certainly see your point...