merge vm docs into guile reference (as yet unfinished)

merge vm docs into guile reference (as yet unfinished)

* doc/ref/compiler.texi: A new empty section on the compiler.

* doc/ref/data-rep.texi: Made to be a section of a chapter instead of an
  appendix. The beginnings of some revision, but not there yet.

* doc/ref/guile.texi: Put the "Data Representation" essay into the new
  "History and Implementation Details" chapter.

* doc/ref/history.texi: New empty section on Guile history.

* doc/ref/libguile-concepts.texi:
* doc/ref/libguile-smobs.texi: Fix up some xrefs.

* doc/ref/vm.texi: New section documenting the VM. Not done yet.