[WIP] implements deferred perspective loading

[titibandit]

This attempts to address #157. This is very much a work in progress, and there already a few problems:

• [ ] upon using persp-state-save, all the deferred are loaded anyway because of the call to with-perspective
• [ ] the test state-save-and-load is broken.
• [ ] new tests are not implemented
• [ ] probably more

I haven't used it much yet, but I intend to do so and try to fix problems as they arise.