Switch maplike/setlike to use infra maps and sets

Fixes #254. Fixes #824.

This switches maplike and setlike interfaces from using a [[BackingMap]]/[[BackingSet]] ES Map/Set to use an Infra map/set instead. As explained in #254 and #824, this makes the interfaces vastly easier to work with in spec-ese (you can directly manipulate an Infra map/set, rather than having to carefully perform the ES algo dance) , and removes some undesired behavior (as currently defined, the algorithms look up and utilize the equivalent methods on Map.prototype and Set.prototype, which can be author-supplied; no implementation does this).

Author: tabatkins