Metadata inference for 'proxy' vars

[vemv]

The following pattern can be sometimes found in Clojure:

(def json-request-format json/json-request-format)

Where

• the def lacks a docstring,
• the def value is a single symbol,
  • therefore its value is known at compile-time
• said symbol can be resolved to a var,
• and that var has a docstring.

...For those cases, the track-state middleware could add :doc metadata for CIDER usage.

Note that this is unobstrusive (non-mutative): users querying (-> #'x meta :doc) would not see our infered value.

We already are doing something similar in #787 (for :style/indent)

So, this pattern can be used for:

• :doc
• :arglists
• :tag
• :style/indent (preempting any inference)
• possibly a few, safe, others.

This can be done for clj and cljs alike.