destructuring :or, fn :pre/:post, deftype field access + map-like dispatch

simon/jolt

General fixes shaken out running clojure.core.cache (66 -> 198 of its assertions):

- Map destructuring applied an :or default only for :keys/:strs/:syms, not a
  direct {x :x} binding — so {x :x :or {x 9}} (and the & {…} kwargs form) ignored
  the default. Apply it for the direct binding too.
- fn didn't implement :pre/:post: a leading conditions map was evaluated as a body
  literal (so % was unbound and (.q %) blew up). Recognize it and assert pre
  before the body, bind % to the result, assert post, return %.
- (.q inst) on a deftype field with no matching method reads the field, like the
  JVM (was "No method q").
- A deftype implementing the clojure.lang collection interfaces now dispatches
  dissoc (without), contains? (containsKey), peek/pop (IPersistentStack), and
  keys/vals (via its Seqable seq) through its methods — they were field-only, so
  core.cache's caches and data.priority-map didn't behave as maps.

JVM-certified corpus rows for each. make test + shakesmoke green.

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