Add support for Set in place of WeakSet

[omrilotan]

Fixes #104

IE11 does not support WeakSet but does support Set. It is still maintained until Aug 2021.
Since the Weakset is created on import, even when the module is loaded and not executed a breakage is caused.

A plain Set implements the necessary API required by this module.

Error: ReferenceError: 'WeakSet' is undefined
User agent string: Mozilla/5.0 (Windows NT 10.0; WOW64; Trident/7.0; rv:11.0) like Gecko (IE11, Win10)

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[omrilotan]

@googlebot I signed it!

[philipwalton]

Thanks!

[Leo555]

IE10 does not support Set。

[omrilotan]

IE10 does not support Set。

IE10 support has ended on January 31, 2020