Improve support for setvar_* attributes

[nspin]

Adds function arguments and struct fields throughout for adding setvar_vaddr, setvar_size, and setvar_id SDF attributes.

Depends on seL4/microkit#338

[Ivan-Velickovic]

Okay I don't see any issues right now just in the destroy functions for the relevant element, call allocator.free on the bytes that you call allocator.dupe with to avoid leaking memory.

[nspin]

I added those allocator.free calls. In doing so, I encountered a bug where those export fn sdfgen_*_destroy functions were being called from Python too early, because their corresponding Python objects were being garbage collected too early. For example, mr was being garbage collected right after a call to sdf.add_mr(mr). So, I added a mechanism for sdf to keep a reference to mr to avoid premature garbage collection.