update docs, clean up VM vestiges, macro docs, fix (/ a b c)

* doc/ref/api-procedures.texi (Compiled Procedures): Fix for API changes.

* doc/ref/compiler.texi (Compiling to the Virtual Machine): Replace GHIL
  docs with Tree-IL docs. Update the bits about the Scheme compiler to
  talk about Tree-IL and the expander instead of GHIL. Remove
  <glil-argument>. Add placeholder sections for assembly and bytecode.

* doc/ref/vm.texi: Update examples with what currently happens. Reword
  some things. Fix a couple errors.

* libguile/vm-i-system.c (externals): Remove this instruction, it's not
  used.

* module/ice-9/documentation.scm (object-documentation): If the object is
  a macro, try to return documentation on the macro transformer.

* module/language/assembly/disassemble.scm (disassemble-load-program):
  Fix problem in which we skipped the first element of the object vector,
  because of changes to procedure layouts a few months ago.

* module/language/scheme/spec.scm (read-file): Remove read-file
  definition.

* module/language/tree-il.scm: Reorder exports. Remove <lexical>, it was
  a compat shim to something that was never released. Fix `location'.

* module/language/tree-il/primitives.scm (/): Fix expander for more than
  two args to /.

* module/system/base/compile.scm (read-file-in): Remove unused
  definition.

* module/system/base/language.scm (system): Remove language-read-file.

* module/language/ecmascript/spec.scm (ecmascript): Remove read-file
  definition.

libguile/vm-i-system.c

@@ -410,12 +410,6 @@ VM_DEFINE_INSTRUCTION (29, toplevel_set, "toplevel-set", 1, 1, 0)
   NEXT;
 }
 
-VM_DEFINE_INSTRUCTION (30, externals, "externals", 0, 0, 1)
-{
-  PUSH (external);
-  NEXT;
-}
-
 
 /*
  * branch and jump