Subrs are RTL programs

Subrs are RTL programs

* libguile/gsubr.c: Define RTL stubs instead of stack VM stubs.
  (SUBR_STUB_CODE, get_subr_stub_code): Adapt to return a uint32_t*
  pointer instead of a SCM value.
  (create_subr): Create RTL procedures instead of stack VM procedures.
  For RTL procedures, the function pointer, name, and generic address
  pointer go inline to the procedure, as free variables.
  (scm_i_primitive_arity, scm_i_primitive_call_ip): New helpers.
  (scm_c_make_gsubr, scm_c_define_gsubr, scm_c_make_gsubr_with_generic)
  (scm_c_define_gsubr_with_generic): Adapt to create_gsubr being renamed
  to create_subr.

  Remove gsubr test code.

* libguile/gsubr.h (SCM_PRIMITIVE_P, SCM_PRIMITIVE_GENERIC_P): Only RTL
  programs can be primitives now.
  (SCM_SUBRF, SCM_SUBR_NAME, SCM_SUBR_GENERIC): These fields are now in
  the RTL free variables, not the object table.

* libguile/programs.c (scm_i_rtl_program_name):
  (scm_i_rtl_program_documentation):
  (scm_i_rtl_program_properties):
  (scm_i_rtl_program_minimum_arity): Implement these appropriately for
  primitives, which lack debugging information.
  (scm_primitive_p, scm_primitive_call_ip): New helpers.

* libguile/snarf.h: Remove static allocation for subrs.  Since there is
  nothing to allocate besides the program itself, which needs runtime
  relocation, static allocation is not a win.

* system/vm/program.scm: Fix up various arity-related things for
  primitives, which don't use ELF arity info.

* test-suite/tests/eval.test ("stack involving a primitive"): Add an
  XFAIL until we get just one VM.