Split ListLen into ListLenU64 and ListLenUsize

The usize one gets used internally for things like
pattern matches. This is both more efficient (means
they don't have to do unnecessary casts) and also
less error-prone due to e.g. comparing length to
capacity, which is usize.

crates/compiler/gen_llvm/src/llvm/lowlevel.rs

@@ -611,7 +611,7 @@ pub(crate) fn run_low_level<'a, 'ctx>(
                 bitcode::STR_WITH_CAPACITY,
             )
         }
-        ListLen => {
+        ListLenU64 => {
             // List.len : List * -> U64
             arguments!(list);
 
@@ -622,6 +622,12 @@ pub(crate) fn run_low_level<'a, 'ctx>(
                 .new_build_int_cast(len_usize, env.context.i64_type(), "usize_to_u64")
                 .into()
         }
+        ListLenUsize => {
+            // List.lenUsize : List * -> usize # used internally, not exposed
+            arguments!(list);
+
+            list_len_usize(env.builder, list.into_struct_value()).into()
+        }
         ListGetCapacity => {
             // List.capacity: List a -> U64
             arguments!(list);