Using RefValue as keys for dicts

[phorward]

Currently, Tokay implements dict as IndexMap<String, RefValue>.

This pull request should change it to IndexMap<RefValue, RefValue> with the important difference, that mutable objects may not be used as keys.

It should be the solution for #79.

Todo:

• [x] Implement dict as IndexMap<RefValue, RefValue>
• [x] Change syntax and semantics of sequences to allow for RefValue as aliases
• [x] ~Improve object hashing to rely on the object~
• [x] Extending test-cases

[phorward]

IMHO this PR is ready for merge. Further improvements follow.