explanations around cleavir based compiler

[Bruno H. <br...@cl...>]

Hi Charles,

Thank you for having added file-level comments to the files.
These comments are very useful, because I wouldn't have guessed
these things.

The comment I loved most is how to make the compiler self-hosting
(in README) - never guessed it would be so easy.

But there are more things that comments could explain:

- Where is readable documentation about the SICL terms? For example,
  while I know that IR means "internal representation", 'HIR' is
  unknown to me.

- What is unphi for? What does the unphi-instruction stand for?

- Is the "edge-split graph" the same thing as defined in
  Definition 2.11 of
  https://dspace.library.uu.nl/bitstream/handle/1874/336410/thesis.pdf ?
  You seem to use an axiomatic definition of "edge-split graph" instead?

- What are the differences between CST and AST? Why does using CST help
  reducing the number of cells of closure vectors?

- What do you mean with "cells are stuck in the strange
  structure of catch-unwind branches"? Can you add an example
  (as source code or IR or byte code, I don't know)?

- When and how do you use graph-form in tools.lisp? What insights does
  it bring?

- What is value-numbering.lisp about?

Bruno