Implement `==`

[ablaom]

Two distributions are == iff they have the same support (but not necessarily same pool) and probabilities on that support agree.

[ablaom]

To clarify, the support consists of the labels explicitly provided at time of construction (which is a subset of the full pool) but may include some probabilities explicitly set to zero. There is not need to check equality for labels that have no corresponding keys in the dictionary because they are implicitly zero.