Warn if new properties are added to PathVisitor Context instances by visit methods

Every `PathVisitor` instance has a class `visitor.Context` such that `Object.getPrototypeOf(visitor.Context.prototype) === visitor`, and the value of `this` within visitor methods is an instance of this `Context` class.

This means that any properties accidentally added to `this` during the visitor method will be thrown away (or become inaccessible) from other visitor methods, probably indicating a bug.

It should be easy to warn about this by checking `Object.keys(context)` after calling the visitor method in `context.invokeVisitorMethod`.