Make evaluation/cache return variables instead of what they point to (for def/defn)

[philomates]

We'd like to allow for different ways to run and render tests from within notebooks.
clojure.test's deftest defines a variable with a :test metadata field present. It would be great to have this at render time, but clerk currently resolves variables returned by def and defn into what they point to. The changes here defer this resolution to the viewer so that we can add a viewer that checks for test meta-data and either hides or executes and shows results. We still don't want to resolve normal vars (that are the result of non-def/defn expression evaluation), so I introduced a wrapped-var to distinguish between the two.

The PR also:

• adds test coverage for read+eval-cached
• breaks up read+eval-cached into smaller functions. Let me if I went too overboard here; many well-named and small functions helps me with code readability but that might be the case for everyone.

open issues to be resolved:

• rendering definition that returns a different var is broken
• rendering deftest results in a string result when it should be render a function

[philomates]

why do functions get no hash, but other non-cachable things do get hashes?

[mk]

because Clojure functions cannot be (de-)serialized.

[mk]

The printing issues should be resolved with dd19391:

Dropping the :render-fn key solved it and the :transform-fn doesn't need to be quoted.

[philomates]

adding this print helped me catch an issue that required adc43c4 and then the 6dea81e follow-up

[philomates]

the results-last-run for (def x 1) will point to {::var-from-def #'x} and then without the (not (::var-from-def value)) we would (intern *ns* #'x {::var-from-def #'x}) which causes the viewer code to stackoverflow when it it tries to display this var.

I'm unsure what the best approach to caching vars that we want to leave unresolved until viewer time (to allow for test running for example). So I just added a skip here. I think that the cache-to-filesystem code still caches the var->value even if it comes from a def, we just never use it.