Removed some useless method aliases in Hamster::Hash

As per GitHub discussion with other contributors, Hamster provides far too many
aliases for methods, unnecessarily bloating the public interface. If individual
programmers want to use their own names for methods, they can easily add aliases
themselves. Hence, in most cases, it is better to just keep the method name which
will likely be the most used.

An exception is where methods are aliased to conform with some other interface --
such as the interface provided by Ruby's built-in collections.

This commit removes Hash#+, #null?, #maximum, #minimum, #uniq, #nub, and

(Regarding #uniq, note that Ruby's built-in Hash does not support #uniq either.)