Generalize readonly

[ThePhD]

Currently, people think that sol::readonly will work on any arbitrary object they set into a table (as per #254). It would be highly beneficial to have a readonly marker for making a whole table read-only, or to at least provide an example in the sol2 examples on how to perform this kind of operation so people understand it better.

[ThePhD]

Unfortunately, we cannot generalize readonly. This would require use to essentially sweep the contents out from under the user, and ultimately screw them over in cases where they set a table as readonly but aren't done working with it or other cases where users would find the behavior unexpected. It also hijacks the __index function, which would make it unintuitive for when a user sets multiple things.

Therefore, it is beneficial to either invest in a table.create_readonly() function or something else. For the time being, documentation will suffice: http://sol2.readthedocs.io/en/latest/api/readonly.html