Explore optionally disabling context population

[Context population](https://www.mistql.com/docs/tutorial/syntax-overview#context-population) is a core part of MistQL that can enable wonderfully readable queries, but occasionally it can lead to weird consequences such as [method overwriting](https://www.mistql.com/docs/tutorial/syntax-overview#method-overwriting) and [non-homogenous arrays](https://www.mistql.com/docs/tutorial/syntax-overview#method-overwriting). We could, as part of MistQL instances, have an option to disable context population and have things exclusively resolve to the @ symbol.

Given that current context is overwritten completely, this'll incur more than a little complexity to get correct, and so I'm not convinced it's a good addition to MistQL without further changes.