make modular kernel tree-shaker agnostic transformers

[sigmundch]

The modular kernel tree-shaker in package:front_end is breaking encapsulation: it contains hard-coded knowledge about methods that may be used by some transformations (like some helper methods for async/await).

Possible solutions to avoid this:

• declare used members in transformers: this might be exposed as methods in kernel.Transformer and kernel.Target that, given a program/CoreTypes, they return a set of members that are expected to exist.
• transformers inject an external declaration for members used: this might not be that easy to for modular transformers though.

/cc @kmillikin

[sigmundch]

We are not planning on using the tree-shaker for some time, marking this as low-priority for now.