Public init for ObjCClass / ObjCProtocol / Enumerator.

These three final subclasses of CommonType / CommonEntity had only
the @Codable-synthesised init(from:), which made them unconstructable
from out-of-tree code. The 0.2.0 surface adds plain memberwise
public init() overloads on each so consumers can build Module
instances programmatically rather than only by decoding existing YAML.