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/system/vm/assembler.scm

@@ -259,6 +259,9 @@
             emit-define!
             emit-current-module
             emit-symbol->string
+            emit-string-utf8-length
+            emit-string->utf8
+            emit-utf8->string
 
             ;; Intrinsics for use by the baseline compiler.
             emit-$car
@@ -1574,6 +1577,9 @@ returned instead."
 (define-scm<-scm-scm-intrinsic define!)
 (define-scm<-thread-intrinsic current-module)
 (define-scm<-scm-intrinsic symbol->string)
+(define-scm<-scm-intrinsic string->utf8)
+(define-scm<-scm-intrinsic utf8->string)
+(define-u64<-scm-intrinsic string-utf8-length)
 
 (define-scm<-scm-intrinsic $car)
 (define-scm<-scm-intrinsic $cdr)