add `nil?' primitive

add `nil?' primitive

* libguile/boolean.c (scm_nil_p): New function.

* libguile/vm-i-scheme.c (nilp, not_nilp):
* libguile/vm-i-system.c (br_if_nil, br_if_not_nil): New instructions.
  Renumber other ops.

* libguile/_scm.h (SCM_OBJCODE_MINOR_VERSION): Increment.

* module/language/assembly/compile-bytecode.scm (compile-bytecode): Add
  support for writing `br-if-nil' and `br-if-not-nil' instructions.

* module/language/assembly/disassemble.scm (code-annotation): Add
  `br-if-nil' and `br-if-not-nil' to the list of branch instructions.

* module/language/tree-il/compile-glil.scm: Add `nil?' to
  `*primcall-ops*'.
  (flatten): Use the new branch instructions for `nil?' conditionals.

* module/language/tree-il/primitives.scm: Add `nil?' to
  `*interesting-primitive-names*', `*effect-free-primitives', and
  `*effect+exception-free-primitives*'.