Branching from discussion in #1588

WIP - master...colinrgodsey:wip-loadmodule-2

Intent

Alter the behavior of JSDesugring to omit unnecessary module load calls in emitted js.

Goals

• Elidable module inits in desugared JS trees based on guarantee of construction
• Elidable inits for modules that have no potential constructor side-effects
• Faster and more dense JS!

Actions

1. Add initializedModules set to JSDesugar#Env
2. Update initializedModules(m) via env for the following trees:
   • Block(...[0], LoadModule(m), ...[1], last)
   • Block(...[0], Apply(LoadModule(m), ...[1]), ...[1], last)
   • More to come....
3. Update genLoadModule with hasInitialized boolean arg. This will refer to either the constructor function or the static reference accordingly. This value should come from Env#initializedModules when possible.
4. Update desugaring to omit side-effect only module inits if initializedModules(m)
5. Analyse LinkedClass for static init expressions (LinkedClass#isPure).
6. Modify module definition to run its init during 'link-time' for pure modules.
7. Modify desugaring to pre-populate initializedModules with the types for pure modules.