Refactor epoch changes to a separate crate (#4785)

* Init epoch changes module

* Initial integration of new epoch changes module for BABE

* Fix all initial compile errors

* rename: digest -> digests

* Fix babe tests

* Bump impl_version

* Fix more test issues

* Remove test flag for tree

It unfortunately won't work for multiple crates.

* Update cargo lock

* Fix duplicate parking_lot version

* Add missing license header

substrate/Cargo.lock

@@ -5805,6 +5805,7 @@ dependencies = [
  "sc-block-builder",
  "sc-client",
  "sc-client-api",
+ "sc-consensus-epochs",
  "sc-consensus-slots",
  "sc-consensus-uncles",
  "sc-executor",
@@ -5832,6 +5833,18 @@ dependencies = [
  "tokio 0.1.22",
 ]
 
+[[package]]
+name = "sc-consensus-epochs"
+version = "0.8.0"
+dependencies = [
+ "fork-tree",
+ "parity-scale-codec",
+ "parking_lot 0.10.0",
+ "sc-client-api",
+ "sp-blockchain",
+ "sp-runtime",
+]
+
 [[package]]
 name = "sc-consensus-manual-seal"
 version = "0.8.0"