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/module/src/low_level.rs

@@ -47,6 +47,7 @@ pub enum LowLevel {
     ListDropAt,
     ListSwap,
     ListAny,
+    ListFindUnsafe,
     DictSize,
     DictEmpty,
     DictInsert,
@@ -225,6 +226,7 @@ macro_rules! higher_order {
             | ListKeepErrs
             | ListSortWith
             | ListAny
+            | ListFindUnsafe
             | DictWalk
     };
 }
@@ -259,6 +261,7 @@ impl LowLevel {
             ListKeepErrs => 1,
             ListSortWith => 1,
             ListAny => 1,
+            ListFindUnsafe => 1,
             DictWalk => 2,
         }
     }

compiler/module/src/symbol.rs

@@ -1066,6 +1066,8 @@ define_builtins! {
         43 LIST_JOIN_MAP_CONCAT: "#joinMapConcat"
         44 LIST_ANY: "any"
         45 LIST_TAKE_FIRST: "takeFirst"
+        46 LIST_FIND: "find"
+        47 LIST_FIND_RESULT: "#find_result" // symbol used in the definition of List.find
     }
     5 RESULT: "Result" => {
         0 RESULT_RESULT: "Result" imported // the Result.Result type alias