Replace "pr" struct fields with "pw" fields

* libguile/struct.h (SCM_VTABLE_BASE_LAYOUT): Layout is a "pr" field.
* module/ice-9/boot-9.scm (record-type-vtable): Record vtable fields are
  writable.
  (<parameter>): "pw" fields.
* module/oop/goops.scm (<class>, %compute-layout): <read-only> fields
  are "pw" underneath.
* module/rnrs/records/procedural.scm (record-type-vtable)
  (record-constructor-vtable, make-record-type-descriptor): Use "pw"
  fields in vtables.
* module/srfi/srfi-35.scm (%condition-type-vtable)
  (struct-layout-for-condition): "pw" fields in vtables.
* test-suite/tests/goops.test:
* test-suite/tests/structs.test: Use "pw" fields only.
* benchmark-suite/benchmarks/structs.bm: Update for make-struct/no-tail,
  to use pw fields, and also to remove useless tests that the compiler
  would optimize away.
* doc/ref/api-data.texi (Vtables): Add a note about the now-vestigial
  permissions character and update documentation.
  (Structure Basics, Meta-Vtables): Update examples.
* libguile/hash.c (scm_i_struct_hash): Remove code that would handle
  opaque/self fields.
* libguile/print.h (SCM_PRINT_STATE_LAYOUT): Use "pw" fields.
* libguile/struct.c (scm_struct_init): Simplify check for hidden
  fields.
* libguile/values.c (scm_init_values): Field is "pw".

libguile/print.h

@@ -53,7 +53,7 @@ do { \
 #define SCM_COERCE_OUTPORT(p) \
   (SCM_PORT_WITH_PS_P (p) ? SCM_PORT_WITH_PS_PORT (p) : p)
 
-#define SCM_PRINT_STATE_LAYOUT "pruwuwuwuwuwpwuwuwurprpw"
+#define SCM_PRINT_STATE_LAYOUT "pwuwuwuwuwuwpwuwuwuwpwpw"
 typedef struct scm_print_state {
   SCM handle;			/* Struct handle */
   int revealed;                 /* Has the state escaped to Scheme? */