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/mono/src/ir.rs

@@ -4164,6 +4164,11 @@ pub fn with_hole<'a>(
 
                     match_on_closure_argument!(ListMap4, [xs, ys, zs, ws])
                 }
+                ListFindUnsafe => {
+                    debug_assert_eq!(arg_symbols.len(), 2);
+                    let xs = arg_symbols[0];
+                    match_on_closure_argument!(ListFindUnsafe, [xs])
+                }
                 _ => {
                     let call = self::Call {
                         call_type: CallType::LowLevel {