dillon1000/react

Support await expressions

Adds support for `await` expressions. We have primarily seen await used inside 
callbacks, not directly within component render logic, but because we construct 
HIR for lambdas it is helpful to be able to model await rather than require 
everyone to rewrite to use the Promise API. Note a subtlety: awaiting a promise 
is a mutative operation, so we a) model it as a Mutate effect and b) avoid DCE 
of await expressions since they may cause side effects. See the test cases for 
examples.