compile @ and @@

compile @ and @@

* libguile/vm-engine.c (vm_run): Add new error case for resolving @ or @@
  references, but there is no such module. Possible if
  module-public-interface returns #f.

* libguile/vm-i-loader.c (link-now): Allow the stack arg to be a sym, as
  before, or a list, indicating an absolute reference. Could be two
  separate instructions, but I'm lazy.

* libguile/vm-i-system.c (late-variable-ref, late-variable-set): As in
  link-now, allow the lazy reference to be a list, for @ and @@.

* module/language/scheme/translate.scm (custom-transformer-table):
  Compile @ and @@, and set! forms for both of them. This will ease the
  non-hygienic pain for exported macros.

* module/system/il/compile.scm (make-glil-var): Translate public and
  private module variable references into glil-module variables.

* module/system/il/ghil.scm (ghil-var-at-module!): New function, resolves
  a variable for @ or @@.

* module/system/il/glil.scm (<glil-module>): Revival of <glil-module>,
  this time with the semantics that it really links to a particular
  module.

* module/system/vm/assemble.scm (<vlink-now>, <vlink-later>): Redefine as
  taking a "key" as the argument, which may be a sym or a list; see the
  notes on link-now for more details.
  (codegen): Compile <glil-module> appropriately. Some duplication here,
  probably could use some cleanup later.