Allows dynamic imports to be loading asynchronously. I know reddit isnt the
best example, since it doesn't fully load, but this reduced the load time from
~7.2s to ~4.8s.
Refactors some of the module loading logic. Both normal modules import and
dynamic module import now share more of the same code - they both go through
the slightly modified `module` function.
Dynamic modules now check the cache first, before loading, and when cached,
resolve the correct promise. This can now happen regardless of the module
loading state.
Also tried to replace some page arenas with call arenas and added some basic
tests for both normal and dynamic module loading.
