rework the vm support for wide strings

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

* libguile/vm-engine.c (vm_error_bad_wide_string_length): New error
  case.

* libguile/vm-i-loader.c (load-unsigned-integer, load-integer)
  (load-keyword): Remove these instructions. The former two are
  obsoleted by make-int64/make-uint64, the latter via make-keyword.
  (load-string): Only handle narrow strings.
  (load-symbol): Only handle narrow symbols. The wide case is handled
  via make-symbol.
  (load-wide-string): New instruction, for wide strings.

* libguile/vm-i-system.c (define): Move here from loaders.c, as now it
  just takes a sym on the stack.
  (make-keyword, make-symbol): New instructions.

* module/language/assembly.scm: Remove removed instructions. No more
  width byte in load-string etc.

* module/language/assembly/compile-bytecode.scm (write-bytecode): Adapt
  to change in instruction set.

* module/language/glil/compile-assembly.scm (glil->assembly): Compile
  define by pushing the sym then emitting (define).
  (dump-object): Dump narrow and wide strings differently. Use
  make-keyword and make-symbol as appropriate.

* module/language/tree-il/compile-glil.scm (flatten): When compiling a
  ref to a primitive (not a call), first see if the primitive is
  actually bound in the root module. (That's not the case with e.g.
  bytevector-u8-ref).

* module/system/xref.scm (program-callee-rev-vars): Don't parse out
  "nexts".

* test-suite/tests/asm-to-bytecode.test ("compiler"): Adapt to bytecode
  format change.

module/system/xref.scm

@@ -35,7 +35,7 @@
                 (progv (make-vector (vector-length objects) #f))
                 (asm (decompile (program-objcode prog) #:to 'assembly)))
             (pmatch asm
-              ((load-program ,nargs ,nrest ,nlocs ,next ,labels ,len . ,body)
+              ((load-program ,nargs ,nrest ,nlocs ,labels ,len . ,body)
                (for-each
                 (lambda (x)
                   (pmatch x