light-client-verifier: Avoid checking signatures multiple times (#1410)

* light-client-verifier: refactor VotingPowerTally by introducing helper methods

* light-client-verifier: avoid checking signatures multiple times

Introduce VotingPowerCalculator::voting_power_in_sets method.

* Update light-client-verifier/src/operations/voting_power.rs

Co-authored-by: Romain Ruetschi <github@romac.me>

* Update light-client-verifier/src/operations/voting_power.rs

Co-authored-by: Romain Ruetschi <github@romac.me>

* comment

* remove vote_power_in and vote_power_in_sets methods

* Revert "remove vote_power_in and vote_power_in_sets methods"

This reverts commit 92ff746.

* reduce diff

* test

* changelog

* docs

---------

Co-authored-by: Romain Ruetschi <github@romac.me>

.changelog/unreleased/breaking-changes/1410-avoid-chucking-signature-multiple-times.md

@@ -0,0 +1,18 @@
+- `[light-client-verifier]` Rework VerificationPredicates and VotingPowerCalculator
+  by introducing methods which check validators and signers overlap at once.
+  The motivation of this is to avoid checking the same signature multiple
+  times.
+
+  Consider a validator is in old and new set.  Previously their signature would
+  be verified twice.  Once by call to `has_sufficient_validators_overlap`
+  method and second time by call to `has_sufficient_signers_overlap` method.
+
+  With the new interface, `has_sufficient_validators_and_signers_overlap` is
+  called and it can be implemented to remember which signatures have been
+  verified.
+
+  As a side effect of those changes, signatures are now verified in the order
+  of validator’s power which may further reduce number of signatures which
+  need to be verified.
+
+  ([\#1410](https://github.com/informalsystems/tendermint-rs/pull/1410))