Add bulk attribute setter operator

[ggreif]

This allows us to write

SVG.marker #=
    [ (SVG.id, "arrow")
    , (SVG.viewBox, "0 0 10 10")
    , (SVG.refx, "5")
    , (SVG.refy, "5")
    , (SVG.markerWidth, "6")
    , (SVG.markerHeight, "6")
    , (SVG.orient, "auto")]
           #+ [SVG.path # set SVG.d "M0 0L10 5L0 10z"]

Importantly, the attribute list can come from any Foldable container.

Stuff to check:

• is there a nicer symbol for the operator (#=)?
• is foldl the correct way to do it?
• is the attribute order relevant (i.e. should we reverse it)?
• squash commits (can be done on merge)
• need to add tests?
• version bump necessary