Refactor paras_inherent::Pallet::enter

[ordian]

As outlined in #849 (comment), we should:

• Change the way we calculate the weight of that function by calculating individually the costs of processing disputes, bitfields, backed candidates and enacting candidates (the latter is done in inclusion: bench enact_candidate weight #5270). We would need to refactor this function such that e.g. processing disputes is contained and can be benchmarked from a single function.
• Remove the calculation of proof_size component of weight - on the Relay Chain the proof size is not limited anyway, so we should just rely on block size limits.

It would also be good to

• Add some type safety, such that processing of bitfields can only be done after processing disputes, etc (e.g. type state pattern or a newtype wrapper).