restructure code a bit, add comparison functions for primitive types and composed types (needed to put values in sets)

structures/struct.js

@@ -7,7 +7,10 @@ function capitalizeFirstLetter(val) {
   return String(val).charAt(0).toUpperCase() + String(val).slice(1);
 }
 
-// [{l: "x", r: Double}, {l: "y", r: Double}] => (Double × (Double × Unit))
+// 'fields' is an array of (name: string, type: Type) pairs.
+// e.g.:
+//   [{l: "x", r: Double}, {l: "y", r: Double}]
+//   results in the type (Double × (Double × Unit))
 export const structType = fields => {
   if (fields.length === 0) {
     return Unit;