light-client-verifier: avoid checking signatures multiple times

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

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+- `[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))