Unload DLLs automatically

[richfitz]

If running interactively, get the generator to store references to the models that have been created with it, set them to destruct with a finaliser and then that means we can tell when a DLL can be unloaded automatically

[richfitz]

This is only an issue when running interactively with odin::odin; we can save copies of the DLL that we create into a general cache and then save copies of models that are generated from that into another cache. Garbage collection then looks like checking that the pointer has been cleared and if so marking the DLL as unloadable, then unloading it

[richfitz]

I forget what the use case is here. I think that it's for people who hit the max loaded dlls limit

[richfitz]

This will likely be an issue with odin.ui

• cache needs to become a ring so that things drop off the end
• then add a finaliser to the R6 class for cleanup, but disable this in package code