Implement List.find

`List.find : List elem, (elem -> Bool) -> Result elem [ NotFound ]*`
behaves as follows:

```
>>> List.find [1, 2, 3] (\n -> n > 2)
Ok 2
>>> List.find [1, 2, 3] (\n -> n > 4)
Err NotFound
```

We implement this as builtin in two phases. First, we call out to a
pure-llvm-lowlevel `ListFindUnsafe` that returns a record indicating
whether a satisfying element was found, and the value of that element
(the value is all null bytes if the element wasn't found). Then, we lift
that record to a `Result` via a standard construction of the can AST.

Closes #1909

compiler/builtins/src/std.rs

@@ -1086,6 +1086,23 @@ pub fn types() -> MutMap<Symbol, (SolvedType, Region)> {
         Box::new(list_type(flex(TVAR1))),
     );
 
+    // find : List elem, (elem -> Bool) -> Result elem [ NotFound ]*
+    {
+        let not_found = SolvedType::TagUnion(
+            vec![(TagName::Global("NotFound".into()), vec![])],
+            Box::new(SolvedType::Wildcard),
+        );
+        let (elem, cvar) = (TVAR1, TVAR2);
+        add_top_level_function_type!(
+            Symbol::LIST_FIND,
+            vec![
+                list_type(flex(elem)),
+                closure(vec![flex(elem)], cvar, Box::new(bool_type())),
+            ],
+            Box::new(result_type(flex(elem), not_found)),
+        )
+    }
+
     // Dict module
 
     // len : Dict * * -> Nat