Expose interfaces, not private package classes.
PerfectHashDictionaryIntIntImpl implements PerfectHashDictionary.
Fix a crash when an empty Dictionary was constructed.
Add a size() method to the dictionary interface.
Use Strings instead of CharSequences everywhere.

Equality is not well-defined for CharSequence and not every CharSequence
is immutable.
Clean up some imports.
Constant time size() for Dictionary.
Add a template and generation script for immutable maps.
Generate map classes for all primitive types.
Add unit tests for ImmutableStringIntTest.