Make grid/aux thinning optional

[rafaqz]

Currently we remove as many grids and aux variables as possible to avoid passing them to the GPU and using all the avaiable registers just to track them. This is less of a problem on modern GPUs, and causes issues on CPUs in shorthand rules where Aux{:x}() is written in code rather than a variable (e.g. #256).

We should add a thin keyword that does this optionally when you want it, but not by default.