Set#delete and Map#delete should return false unless a deletion occurred.

Relates to #298.

Per:
 - https://github.com/paulmillr/es6-shim/commit/b90f0a0fe92074af2f5831b713de7e2dfe78c18b#commitcomment-8326164
 - https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-set.prototype.delete
 - https://people.mozilla.org/%7Ejorendorff/es6-draft.html#sec-map.prototype.delete
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