Extract iD.actions.DiscardTags
Clean up and simplify iD.svg.Lines
Clean up and simplify iD.svg.Areas
Reduce calls to iD.svg.TagClasses

For fills and shadows, style changes can happen only when
a new version of the feature is produced. If we include a
version number in the key, we need only call TagClasses on
the enter selection.

We cannot apply the same optimization for strokes, since
stroke style can depend on the tags of parent relations
as well.
Optimize vertex rendering

* Cache icon
* Append rather than insert
* Do fewer things on update
* Don't create a fill unless needed
* Don't apply tag and member classes (never used)
* Drop down to raw setAttribute (d3 is slow :trollface:)