Command modules are easy to leak memory with

The implementation of "command" modules in `Linker` will create a new `Instance` each time an export is invoked. This works well implementation-wise but it's also easy to leak memory and resources unintentionally. No `Instance` is actually deallocated until the `Store` is dropped, which means that commands are a vector for repeatedly creating instances by accident without ever actually deleting them.

I'm not really sure what the best way to handle this would be, but we should probably either document this or look into a fix!

cc @sunfishcode

Start by reading the Linker command-module implementation and the Instance and Store lifecycle described in the issue. Determine whether the appropriate outcome is documentation or a fix; done means either clearly documenting the retention behavior or preventing unintended accumulation of instances and resources.

Written by the indexing model from the issue text.