Cleanup

[szabo137]

During the restructuring of QED.jl, some little cosmetic issues were introduced (or copied):

• check for obsolete dependencies (in deps and extras)
• Drop usage of Suppressors in the tests and use TestImpl instead.
• rename algebraic_objects
• use import Pack and Pack.func(...) = ... instead of using Pack: func and func(...) = ... for implementing an interface.
• move all import statements to src/QEDcore.jl
• Drop usage of SimpleTraits.jl (maybe after Refactor: FourMomentum interface QEDbase.jl#59)
• refactor src/particles/spinors.jl (will be done in a separate issue)
• reexport base_state (?)
• remove mul because it is redundant if Base.* is implemented anyway.

[AntonReinhard]

As far as I can see there's only one import left, which is import Base: * and I would keep that where it's needed to be less confusing. Directly overloading Base.*() sadly doesn't work, so it has to be imported.
There are also no usings left in src/ outside of QEDcore.jl