Remove extra space between key and value.

1. Extra space removed.
2. Using acc.push_str() instead of str::push_str
3. Update test to reflect representation change.
Add str representation for HashSet.
Add test: test_hashset()
Use acc.push_str() instead of str::push_str(..)

Also added some whitespace to enhance readabilty.
Fix errors in test_hashset().

All tests now pass.