Support for bools. Also fix fallible/infallible mixup for ctors.

cranelift/isle/isle_examples/test4.isle

@@ -1,4 +1,5 @@
 (type u32 (primitive u32))
+(type bool (primitive bool))
 (type A (enum (A1 (x u32))))
 
 (decl Ext1 (u32) A)
@@ -6,6 +7,9 @@
 (extern extractor Ext1 ext1)
 (extern extractor Ext2 ext2)
 
+(decl C (bool) A)
+(extern constructor C c)
+
 (decl Lower (A) A)
 
 (rule
@@ -14,4 +18,4 @@
       a
       (Ext1 x)
       (Ext2 =x)))
-  a)
+  (C #t))