Hard-deprecate Reducer struct type alias (#1478)

* Hard-deprecate `Reducer`

One step from our roadmap: pointfreeco/swift-composable-architecture#1477

* wip

(cherry picked from commit b64017264850a84b897c90641dd1f61eb1a1f566)

# Conflicts:
#	Sources/ComposableArchitecture/Internal/Deprecations.swift
#	Tests/ComposableArchitectureTests/EffectTests.swift

Author: stephencelis