* Core: lazy-load worlds in unpickler
this should hopefully fix customserver's memory consumption
* WebHost: move imports around to save memory in MP
* MultiServer: prefer loading _speedups without pyximport
This saves ~15MB per MP and speeds up module import if it was built in-place.
* Tests: fix tests for changed WebHost imports
* CustomServer: run GC after setup
* CustomServer: cleanup exception handling
black-sliver