Do fewer linear searches of element stacks

[kmcallister]

e.g. the open elements or active formatting elements. We search these when looking for elements in various scopes, and #77 adds more.

We could track open elements with a bitvector for each scope. Checking if one of a set of elements is open in a particular scope would be a simple bitmask test. We can choose bit patterns to match the static atom indices as well.

We should find an abstraction that makes it hard to screw up the book-keeping.

[BurgundyWillow]

I wanted to know if this is still an issue, and I would like to work on this. In #77 I did not see any pull requests/commits resolving this specific request, and wanted to confirm the same.