feat(identity): rename exported uri

In order to export the URI of any instance, they should be uniquely named. While URIs should
typically not be used directly, they may be valuable when writing adaptors.

BREAKING CHANGE: URI is now exported from Identity as MAYBE_URI (constant) and MaybeURI (type)

src/instance/Identity.ts

@@ -6,8 +6,11 @@ declare module '../HKT' {
   }
 }
 
-export const URI = 'Identity'
-export type URI = typeof URI
+const URI = 'Identity'
+type URI = typeof URI
+
+export const IDENTITY_URI = URI
+export type IdentityURI = URI
 
 /**
  * A simple wrapper around a value which provides no special behavior.