Add to_str() for HashMaps, and some basic tests as well.
Removed test_hashmap() for the time, being. All tests pass with make check.
All tests, including newly added test_hashmap() pass. The empty Hash Table doesn't need to be mutable.
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.
auto merge of #6414 : samebchase/rust/experimental, r=Aatch

Implemented to_str() for HashMap and HashSet
Added tests.
Minor formatting and stylistic cleanups.