Remove depenendcy on sc-consensus-slots

[tomusdrw]

This cleans up [dependencies] section of all primitives from client deps 🎉 :)
We are only left with [dev-dependencies] now (I guess that requires moving the tests to separate test crate, cc @ben).

[Demi-Marie]

LGTM!

[gnunicorn]

LGTM

[tomusdrw]

I've tried to apply @niklasad1 suggestions, but I think omitting lifetime is not yet supported for traits:

   Compiling sc-executor v2.0.0 (/home/builds/remote-builds/substrate2/client/executor)
error[E0106]: missing lifetime specifier
   --> primitives/consensus/common/src/lib.rs:191:18
    |
191 |     const SLOT_KEY: &[u8];
    |                     ^ expected lifetime parameter

error[E0106]: missing lifetime specifier
   --> primitives/consensus/common/src/lib.rs:199:18
    |
199 |     const SLOT_KEY: &[u8] = b"aura_slot_duration";
    |                     ^ expected lifetime parameter

error: aborting due to 2 previous errors

[niklasad1]

I've tried to apply @niklasad1 suggestions, but I think omitting lifetime is not yet supported for traits:

Sorry, my suggestions were wrong I thought that references in associated constants were deduced as static. Clearly not, see rust-lang/rust#51370