start at documenting the compiler

start at documenting the compiler

* doc/ref/api-evaluation.texi: Add documentation for the standard
  compilation interface, and some notes about compiled files.

* doc/ref/api-procedures.texi (Compiled Procedures): A stub at
  documenting compiled procedures.

* doc/ref/compiler.texi (Compiling to the Virtual Machine): Flesh out
  with some structure, though much of the text remains to be written.
  This stuff is hard to write!