misbehavior: report multiple offenses per validator as necessary (#2222)

* use proper descriptive generic type names

* cleanup

* Table stores a list of detected misbehavior per authority

* add Table::drain_misbehaviors_for

* WIP: unify misbehavior types; report multiple misbehaviors per validator

Code checks, but tests don't yet pass.

* update drain_misbehaviors: return authority id as well as specific misbehavior

* enable unchecked construction of Signed structs in tests

* remove test-features feature & unnecessary generic

* fix backing tests

This took a while to figure out, because where we'd previously been
passing around `SignedFullStatement`s, we now needed to construct
those on the fly within the test, to take advantage of the signature-
checking in the constructor. That, in turn, necessitated changing the
iterable type of `drain_misbehaviors` to return the validator index,
and passing that validator index along within the misbehavior report.

Once that was sorted, however, it became relatively straightforward:
just needed to add appropriate methods to deconstruct the misbehavior
reports, and then we could construct the signed statements directly.

* fix bad merge
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