Allow non-environment "environments"

In R today, every frame has an environment. However, the likes of `dplyr` and `ggplot2` have shown that there's value in allowing other types of data to act as an environment (akin to using `within()`).

Although this would be "non-standard", this would allow calling `eval()` on an expression using an "environment" defined by a `data.frame`, `list`, or other data object. This would be similar to `attach`, but with mutable access to data.

This would also allow someone to implement a `get` and `assign` method for their data to allow it to operate as its own "environment" as part of a call stack, which might make it easier to implement `dplyr`-y interfaces.