JavaScript Backend: Implement the Compactor

Description

GHCJS's Compactor is a memory optimization that shortens the length of binder names in the generated JS code. This reduces file size and is necessary for certain programs to run.

This is the tracking issue to implement the compactor in the JS Backend, post MR !9133 (merged)

Impacted Tests

Tick these off as you go:

• recomp007 passes
• recompChangedPackage passes
• recompTHpackage passes
• T6145 passes
• literals passes
• apirecomp001 passes
• PartialDownsweep passes
• parsed passes
• T13350 passes
• T13168 passes
• bug1465 passes
• T20242 passes
• T21336b passes (this one is in base/tests/IO)
• seward-space-leak passes