Implement permissions checks in Interpreter

[cpcallen]

Tracking bug for implement permissions checks in the server Interpreter.

Prerequisites:

• Finalise Permissions Model #392.
• Rewrite core classes to use WeakMaps #404.

To Do:

• Implement controls() or similar.
• Implement object mutability limits (only mutable by controllers(s)).
• Implement normal property writability limits (only writable by controller(s)).

Later:

• Implement object readability attribute? (Needed to protect $.userDatabase keys.)
• Implement property readable attribute or similar?
• Implement property heritable attribute or similar.

[cpcallen]

We could just hide all the sensitive information in $.userDatabase inside a closure. We'd need to have a way (for sufficiently authorised users) to extract the information manually so it could restored after a dump/reload, but that would obviate the immediate need for non-readable objects, AFAICT.