Contextual implementation

[azidar]

Separable PRs:

• [x] Split InstanceSpec numbering into separate PR
• [ ] Change to requirement of having IsInstantiable explicit vs implicit

TODO(@azidar):

• [x] Scaladoc for Proxy.scala
• [x] Scaladoc for Lookupable.scala
• [x] Scaladoc for Context.scala
• [x] Scaladoc for ModuleClone.scala
• [x] Scaladoc for ModuleTransparent.scala
• [x] Scaladoc for ModuleMock.scala
• [x] Scaladoc for ModuleDefinition.scala
• [x] Scaladoc for package.scala