move to lib

[cryptoAtwill]

To migrate main.rs to use both main.rs and lib.rs, reasons are as following:

1. Currently we are using main.rs, but it's empty and it leads to a lot of clippy dead code warning. Moving to lib.rs will resolve these warnings.
2. When writing integration tests for SubnetManager, it needs to import modules from ipc-agent, hence it's only possible if we change to lib.
3. Seems doc tests are compiled only with lib, see Doctests don't work in bin targets, non-public items rust-lang/rust#50784, I could be wrong, but somehow cargo test does not trigger the doc tests, with lib it's just easy.

Since there is the chance, I also fixed a few clippy warning, so that we are now warning free!

[adlrocha]

LGTM. Let's get a 👍 from @hmoniz that he is OK with this approach too and let's get it merged.

[hmoniz]

LGTM. Assuming you'll address Akosh's comments.