Simplify some code in ScriptTask::load.
auto merge of #4444 : Ms2ger/servo/load-simplify, r=jdm
Create a replacement for Cell<SM primitive>. Fixes #4337.
auto merge of #4401 : jdm/servo/heapsm2, r=Manishearth

We still will need a lint to ban any further uses, but that can be a followup.