Support incomplete list for sorter and renamer

[haberdashPI]

This addresses the proposal in #329 to make it possible to handle palettes a little more cleanly:

Specifically the missing piece this adds is to handle the case where sorter and renamer are provided only a partial enumeration of values.

Then, to quote @piever, the old:

# `a` and `b` entries get specified colors, for the other entries it's data-dependent
palette = (color=["a" => "red", "b" => "blue", "green", "black"],)

can now be achieved with

mapping(color=v => sorter(["a", "b"])
palette = (color=["red", "blue", "green", "black"],)

[piever]

About the implementation, instead of overloading map, I was thinking that one could try the following. renamer and sorter, rather than returning a Sorted object, would return a

struct Renamed{S, T}
    original::S
    value::T # maybe `Union{T, Nothing}` is cleaner, otherwise just a random one if the correct one is not found
    idx::UInt32 # use `0` if `original` is not found in `uniquevalues`
end

object. Then, this gets all simplified to something reasonable using rescale at the very end.

[haberdashPI]

Ah yes, I see, that is probably a better way to do it!

Above is just an initial commit/sketch. It's untested, and I haven't added any new tests yet.