Proto/validate: check slot unicity in denunciation evidences

What

Denunciations where one of the evidence operations contains a committee with duplicate slots are now invalid.

Why

Otherwise it would be possible to craft a valid denunciation that has max operation size.

How

Manually testing the MR

Check that the new tests fail before the last commit and succeed after it:

dune exec src/proto_alpha/lib_protocol/test/integration/consensus/main.exe -- -f test_double_attestation.ml
dune exec src/proto_alpha/lib_protocol/test/integration/consensus/main.exe -- -f test_double_preattestation.ml