GRANDPA justifications: equivocation detection primitives (#2295) (#2297)

* GRANDPA justifications: equivocation detection primitives

- made the justification verification logic more customizable
- added support for parsing multiple justifications and extracting
  equivocations
- split the logic into multiple files
- split the errors into multiple enums

bridges/primitives/header-chain/tests/tests.rs

@@ -0,0 +1,7 @@
+mod justification {
+	mod equivocation;
+	mod optimizer;
+	mod strict;
+}
+
+mod implementation_match;