Note re leaving out call/cc and apply
Exercise 6.2, the `list` primitive
Factor out interpreter/compiler scaffolding

The pretreatment doesn't change (much?) from chapter 6 to chapter 7,
and maybe beyond.

Notably: environment (activation frames, globals etc.) and closures
(though they are polymorphic).
First chapter7 compiler

This one compiles to lists of thunks. I've put in a simple stepper to
try it out:

    (compile+run '(+ 1 2))