Allow string->utf8 to constant-fold

* module/language/tree-il/primitives.scm (*interesting-primitive-names*):
(*primitive-accessors*): Add string->utf8, utf8->string, and
string-utf8-length.
(primitive-module): New public function, moved here from (language
tree-il compile-bytecode).

* module/language/tree-il/compile-bytecode.scm: Use primitive-module
from (language tree-il primitives).

* module/language/tree-il/peval.scm (peval): A bugfix: load primitives
from their proper module.  Allows bytevector primitives to fold.

* module/language/cps/guile-vm/reify-primitives.scm:
* module/language/cps/effects-analysis.scm:
* module/language/cps/types.scm
* module/language/tree-il/primitives.scm:
* module/language/tree-il/cps-primitives.scm:
* module/language/tree-il/effects.scm (make-effects-analyzer):
Add string->utf8, utf8->string, and string-utf8-length.

* module/language/tree-il/compile-cps.scm (string->utf8)
(string-utf8-length, utf8->string): New custom lowerers, including type
checks and an unboxed result for string-utf8-length.

* module/system/vm/assembler.scm:
* libguile/intrinsics.h:
* libguile/intrinsics.c: Because string-utf8-length returns an unboxed
value, we need an intrinsic for it; go ahead and add an intrinsic for
string->utf8 and utf8->string too, as we will likely be able to use
these in the future.

module/language/cps/guile-vm/reify-primitives.scm

@@ -26,6 +26,8 @@
 
 (define-module (language cps guile-vm reify-primitives)
   #:use-module (ice-9 match)
+  #:use-module ((language tree-il primitives)
+                #:select ((primitive-module . tree-il:primitive-module)))
   #:use-module (language cps)
   #:use-module (language cps utils)
   #:use-module (language cps with-cps)
@@ -36,47 +38,7 @@
   #:export (reify-primitives))
 
 (define (primitive-module name)
-  (case name
-    ((bytevector?
-      bytevector-length
-
-      bytevector-u8-ref bytevector-u8-set!
-      bytevector-s8-ref bytevector-s8-set!
-
-      bytevector-u16-ref bytevector-u16-set!
-      bytevector-u16-native-ref bytevector-u16-native-set!
-      bytevector-s16-ref bytevector-s16-set!
-      bytevector-s16-native-ref bytevector-s16-native-set!
-
-      bytevector-u32-ref bytevector-u32-set!
-      bytevector-u32-native-ref bytevector-u32-native-set!
-      bytevector-s32-ref bytevector-s32-set!
-      bytevector-s32-native-ref bytevector-s32-native-set!
-
-      bytevector-u64-ref bytevector-u64-set!
-      bytevector-u64-native-ref bytevector-u64-native-set!
-      bytevector-s64-ref bytevector-s64-set!
-      bytevector-s64-native-ref bytevector-s64-native-set!
-
-      bytevector-ieee-single-ref bytevector-ieee-single-set!
-      bytevector-ieee-single-native-ref bytevector-ieee-single-native-set!
-      bytevector-ieee-double-ref bytevector-ieee-double-set!
-      bytevector-ieee-double-native-ref bytevector-ieee-double-native-set!)
-     '(rnrs bytevectors))
-    ((atomic-box?
-      make-atomic-box atomic-box-ref atomic-box-set!
-      atomic-box-swap! atomic-box-compare-and-swap!)
-     '(ice-9 atomic))
-    ((current-thread) '(ice-9 threads))
-    ((class-of) '(oop goops))
-    ((u8vector-ref
-      u8vector-set! s8vector-ref s8vector-set!
-      u16vector-ref u16vector-set! s16vector-ref s16vector-set!
-      u32vector-ref u32vector-set! s32vector-ref s32vector-set!
-      u64vector-ref u64vector-set! s64vector-ref s64vector-set!
-      f32vector-ref f32vector-set! f64vector-ref f64vector-set!)
-     '(srfi srfi-4))
-    (else '(guile))))
+  (tree-il:primitive-module name))
 
 (define (primitive-ref cps name k src)
   (with-cps cps
@@ -338,6 +300,7 @@
       string->symbol
       symbol->keyword
       symbol->string
+      string-utf8-length string->utf8 utf8->string
       class-of
       scm->f64
       s64->u64 s64->scm scm->s64