[NCC-E008263-4VP] Unchecked Accesses to Data Structures

[conradoplg]

In multiple instances, dictionary are accessed directly, which leads to panics if the key is not present. See report for more details.

Also, in aggregate(), we should make sure that the 3 arguments have matching sets of identifiers.

Fix those:

• It's probably better to avoid indexing directly altogether and use .get() instead, handling the error. Try enabling the indexing_slicing lint (possibly with an exception in test code)
• Add a validation in aggregate() to make sue the identifier sets are consistent (signing_package.signing_commitments and signature_shares should have equal keys, and they should be a subset of pubkeys.signer_pubkeys)