Add support for IPMap type

[joshgodsiff]

Hey, cool library 😄

I have a use case that's fairly similar to the IPSet type, except I need to map IPNetworks to particular values, while merging CIDRs together if the values are the same. As far as I can tell, this isn't currently possible in the library, but it seems like something that would be broadly useful, and not too difficult to implement.

As an example, I'd like something that worked like:

>>> IPMap({'192.0.2.0': 'foo'}) | IPMap({'192.0.2.1': 'foo'})
IPMap({'192.0.2.0/31': 'foo'})

>>> IPSet({'192.0.2.0': 'foo'}) | IPSet({'192.0.2.1': 'bar'})
IPMap({'192.0.2.0': 'foo', '192.0.2.1': 'bar'})