Check range errors at the protocol level

[fjarri]

There are multiple places at the protocol level where we can't be sure about the self-consistency of party idx and the total number of parties. e.g.:

• the number of parties should be non-zero (maybe even >1)
• the party index must be within the number of parties
• the party index should not coincide with the current party's index (e.g. in verify_direct_message()/verify_broadcast())
• the consistency of the party index/number of parties obtained from different sources (e.g. on creation of presigning::Round1 we get those as separate arguments, and in a KeyShare object)
• the correct size and excluded index in the HoleVec objects passed to finalize_to_result()/finalize_to_next_round() methods

These should be asserted.

[fjarri]

We are using maps/sets keyed by an ID type now, so this issue is largely invalidated. To close it completely we need to enable clippy::indexing_slicing lint.