Represent terms with an opaque data type

Partial progress here

Currently, Redex terms are stored as S-expressions. If we stored them as opaque values, destructurable only through pattern-matching, it would make possible some potentially important optimizations:

• caching the results of pattern-matching (this has the potential to improve the big-O of traversals)
• using Racket syntax objects to store the actual terms, enabling binding operations to use sets-of-scopes to perform mass renaming quickly (this should improve the big-O of traversals of syntax with binding)

Furthermore, it discourages the use of non-Redex operations on Redex terms. This is potentially a good thing, because many of the operations are unhygenic in surprising ways (for example equal? is not safe to use on terms in a language with binding).

The branch I linked above has significant progress towards this representation change (after which, implementing the above optimizations could be done in a local and contained fashion). I believe I've put the boundaries of the representation change in the right place, or almost the right place, and all of the unit tests pass, last I checked. Some models need to be updated; mainly to remove cases where they they treat terms as S-expressions.

However, once all of the example artifacts are updated, we can make this a largely backwards-compatible change. S-expressions should already be automatically coerced to terms, and (at least for a while, with the ability to opt-out) terms can be automatically coerced to S-expressions once they are put into the user's hands.

I was hoping to get this done myself (and I still might!), but I didn't want my progress to languish without at least making a couple notes here.

Start by reviewing the linked new-representation branch and the existing unit tests, which the issue says currently pass. Then identify the example artifacts that still treat terms as S-expressions. Done means updating those artifacts and completing the representation change while preserving the described coercion behavior.