Tests updated

 * Added tests for `Set`
 * Refactored tests for `Map`
  * Revived not running tests
  * Fixed tests that used value of the type-providing (empty) key 
  * Unified tests

This pull request is independent, but is ready to be applied atop #69.

Tests for `split` and `splitLookup` fails, because current implementation of them is invalid. Fixed implementations come in #69.