Add bind-optionals instruction

* doc/ref/vm.texi (Function Prologue Instructions): Document new
  instruction.
* libguile/jit.c (compile_bind_optionals): New compiler.
* libguile/vm-engine.c (VM_NAME): New interpreter.
* module/system/vm/assembler.scm (opt-prelude): Emit bind-optionals as
  appropriate.
* module/system/vm/disassembler.scm (define-stack-effect-parser)
  (code-annotation): Handle bind-optionals.

doc/ref/vm.texi

@@ -807,6 +807,12 @@ will signal an error if an unknown key is found.
 A macro-mega-instruction.
 @end deftypefn
 
+@deftypefn Instruction {} bind-optionals f24:@var{nlocals}
+Expand the current frame to have at least @var{nlocals} locals, filling
+in any fresh values with @code{SCM_UNDEFINED}.  If the frame has more
+than @var{nlocals} locals, it is left as it is.
+@end deftypefn
+
 @deftypefn Instruction {} bind-rest f24:@var{dst}
 Collect any arguments at or above @var{dst} into a list, and store that
 list at @var{dst}.
@@ -814,8 +820,7 @@ list at @var{dst}.
 
 @deftypefn Instruction {} alloc-frame c24:@var{nlocals}
 Ensure that there is space on the stack for @var{nlocals} local
-variables, setting them all to @code{SCM_UNDEFINED}, except those values
-that are already on the stack.
+variables.  The value of any new local is undefined.
 @end deftypefn
 
 @deftypefn Instruction {} reset-frame c24:@var{nlocals}