SortedDict missing mergewith

[StephenVavasis]

SortedDict currently does not implement the Base functions mergewith and mergewith!. So mergewith of two SortedDicts treats them both as AbstractDict and returns a Dict instead of a SortedDict.

Additional missing methods from Base.

1. map!(f, values(s::SortedDict))
2. first(a,n) and last(a,n) where a is a sorted container and n is an integer.
3. filter for SortedDict and SortedMultiDict
4. 'pairs' for SortedDict and SortedMultiDict