Added make-nil instruction to VM and use it for Emacs' nil in the compiler.

Added make-nil instruction to VM and use it for Emacs' nil in the compiler.

* doc/ref/vm.texi: Document new instruction.
* libguile/vm-i-system.c: Add it to the VM.
* module/language/assembly.scm: Compile (const %nil) to (make-nil) assembly.
* module/language/glil/decompile-assembly.scm: Handle (make-nil)
* module/language/elisp/compile-tree-il.scm: Use (const %nil) for nil.