tests: exercise array getters

crates/wiggle/tests/arrays.witx

@@ -15,3 +15,22 @@
     (result $error $errno)
   )
 )
+
+(typename $pair_ints
+  (struct
+    (field $first s32)
+    (field $second s32)))
+
+(module $array_traversal
+  (@interface func (export "sum_of_element")
+    (param $elements (array $pair_ints))
+    (param $index (@witx usize))
+    (result $error $errno)
+    (result $sum s32))
+
+  (@interface func (export "sum_of_elements")
+    (param $elements (array $pair_ints))
+    (param $start (@witx usize))
+    (param $end (@witx usize))
+    (result $error $errno)
+    (result $sum s32)))