Support read-only generation and freeze operation

[andylowry]

Read-only generation would allow creation of a parser in which all write methods are omitted entirely, which is probably suitable for many use-cases, and results in a significantly simpler API.

Freezing would be made available via the overlay adapter class. It would result lock the targeted overlay object in its current state; all mutation operations would throw illegal state exception. Freezing could be recursive, so that all descendant overlay objects would also be frozen. This could optionally include overlays reachable via reference.

A parser option would recursively freeze the entire model after parsing.