Verify Ancestry between Headers (#3963)

* Create module for checking ancestry proofs

* Use Vec of Headers instead of a HashMap

* Move the ancestry verification into the lib.rs file

* Change the proof format to exclude `child` and `ancestor` headers

* Add a testing function for building header chains

* Rename AncestorNotFound error to InvalidAncestryProof

* Use ancestor hash instead of header when verifying ancestry

* Clean up some stuff missed in the merge