Finality Verifier Pallet (#629)

* Add skeleton for `pallet-finality-verifier`

* Sketch out implementation for importing finality proofs

* Get pallet compiling

* Introduce skeleton for mock runtime

* Start using real Grandpa types in finality pallet

* Redefine types in header chain primitives crate

* Implement HeaderChain for Substrate bridge pallet

* Plug Substrate Bridge Pallet into verifier mock

* Fix compilation of `header-chain` primitives

* Start writing to base pallet storage

* Add first "cross-pallet" test

* Move keyring primitives used in tests to shared crate

* Stop pulling `std` deps into `no_std` builds

* Revert "Stop pulling `std` deps into `no_std` builds"

This reverts commit f74dd660652f98b7336936d1534a4e63cc9169a5.

* Revert "Move keyring primitives used in tests to shared crate"

This reverts commit b774fa730b2cdc40545afff308a66b0840266001.

* Use new SS58Prefix type in mock

* Start using `bp-test-utils` in finality pallet

* Start using real justification code

* Get a test working with real justification verification

* Add basic tests for invalid proofs

* Get rid of AncestryProof config type

* Add error types to transaction outcome

* Bound number of headers allowed in a single ancestry proof

* Disallow invalid authority sets

* Remove unused items

* Add some documentation

* Get rid of Clippy warnings

* Rename BaseHeaderChain to TransactionVerifier

* Remove unused code

* Make dummy trait implementations more generic

* Fix more Clippy complaints

* Update tests to use fix for duplicate headers

* Fix benchmarking compilation

* Rename TransactionVerifier to InclusionProofVerifier