Compiler output for debugging and inspection

The compiler should support debug output, printing expressions so we can see the effects of optimisations and plan improvements. For PureLang we have `compiler/parsing/sexp` - at least ThunkLang should implement the same, and this can be plumbed through the compiler with some appropriate flag.

Start by reading compiler/parsing/sexp and the existing ThunkLang compiler entry points to understand how PureLang handles debug output. Determine the appropriate compiler flag and the scope of expression printing, then define how output should expose optimisation effects and verify it with the relevant compiler checks.

Written by the indexing model from the issue text.