Enable more clippy lints

[thomaseizinger]

There are a couple of interesting lints in clippy that are allow by default but might be useful for us:

Wrong integer arithmetic:

• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#cast_possible_truncation
• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#cast_possible_wrap
• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#cast_precision_loss
• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#cast_sign_loss

Invalid From implementation:

• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#fallible_impl_from

Println statements:

• https://rust-lang-nursery.github.io/rust-clippy/master/index.html#print_stdout
  There should be no println statements in the code base but only logger statements.

Use of clone for Arcs and Rcs:
- https://rust-lang-nursery.github.io/rust-clippy/master/index.html#clone_on_ref_ptr

[LLFourn]

"open source" added to force discussion.

[D4nte]

IMHO: agree for opensource.

[D4nte]

Agreed for all.
Please note, type inference does not work as well with Arc::clone() and hence we may need to deactivate the warning locally in some places.

[bonomat]

It seems like clippy does not support a proper config file right now and we need to fall back to passing extra lints as arguments ala:
clippy -- -W clippy::clone_on_ref_ptr -D warnings

More to read: rust-lang/rust-clippy#3164

[bonomat]

I opt for not using clone_on_ref_ptr as we would need to make too many exceptions. However, we should make it best practice to use Arc::clone over .clone() where possible.