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/inc_dec.rs

@@ -531,7 +531,8 @@ impl<'a> Context<'a> {
                     | ListKeepIf { xs }
                     | ListKeepOks { xs }
                     | ListKeepErrs { xs }
-                    | ListAny { xs } => {
+                    | ListAny { xs }
+                    | ListFindUnsafe { xs } => {
                         let borrows = [function_ps[0].borrow, FUNCTION, CLOSURE_DATA];
 
                         let b = self.add_dec_after_lowlevel(arguments, &borrows, b, b_live_vars);