Added the empty() method.

The empty method removes all key/value pairs from the mapping. As of
now, it works by deleting all the keys in the internal store… maybe it
would be better to just create a completely new store? I think this way
is conceptually cleaner, though.
Merge branch 'empty' into develop
Added the filter() method.

The filter method lets us filter out a subset of the mapping. It
returns a new mapping, and the original mapping isn’t modified.
Merge branch 'filter' into develop
Added the some() method.

The some method lets us check if at least one key/value pair meets a
certain criteria.
Merge branch 'some' into develop
Added the all() method.

The all method returns true if all of the key/value pairs in the
mapping meet some criteria.
Merge branch 'all' into develop
Added the one() function.

The one function tells us if exactly one key/value pair in the mapping
satisfies some criteria.
Merge branch 'one' into develop
Merge branch 'develop'