Add expression element identifier prettyprinter annotations

[kquick]

Now that what4 is changed to use prettyprinter output with the ability to annotate pretty-printed elements, it may be useful to create a concrete annotation type to identify the element type for rendering-time formatting (see https://github.com/GaloisInc/what4/pull/77#discussion_r530023412).

[robdockins]

What sorts of annotations do you have in mind?

[kquick]

We can have annotations for identifying literal values, operators, and the like that would allow for various highlighting or other formatting choices during the rendering phase.