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/bitcode/src/main.zig

@@ -52,6 +52,7 @@ comptime {
     exportListFn(list.listSetInPlace, "set_in_place");
     exportListFn(list.listSwap, "swap");
     exportListFn(list.listAny, "any");
+    exportListFn(list.listFindUnsafe, "find_unsafe");
 }
 
 // Dict Module