NatSpec inheritance and transclusion

[ItsNickBarry]

NatSpec comments are generally needed on both external interfaces and internal contracts. External contracts inherit their comments from external interfaces. In cases where internal functions correspond to external functions (in accordance with the layers pattern), these comments should be identical.

We need a script (or at least a test) to make sure that no comments are missing, and that they are consistent across contracts. Ideally it would also find cases where inheritdoc can be used for overridden internal functions.