v0.15: HashmapE / Dictionary

Now you can read/write dictionaries (HashmapE) from/to cells:

• Use LoadDict and TryLoadDict to load Cell with dict data (or something that has been stored as dict data) from Slice, then call ParseDict to load actual dict data from that Cell.
• Use LoadAndParseDict, TryLoadAndParseDict to load and immediately parse dictionary.
• Use StoreDict overloads to store Cell with serialized dict data or serialize-and-store dict.

Check DictTests.cs for some samples.