Have activeContext.sealed use a Set rather than a hashmap

[cwebber]

Assuming #289 is pulled in, activeContext.sealed should use a Set. Currently it's using the "hashmap as a set" hack because of cache serialization issues. Ie, shenanigans like so would happen:

Set { '[', 'o', 'b', 'j', 'e', 'c', 't', ' ', 'S', ']' }

[dlongley]

This is not part of the implementation, closing.