Use a unique queue for pending analyses

[Bastacyclop]

Fixes #239
Alternative to #250

[mwillsey]

Unfortunately, you'll have to implement (De)Serialize because you are making a new data structure inside EGraph.

[Bastacyclop]

Right, I didn't use make nits. It should be good now, although I do get the following errors:

Found invalid urls in struct.EGraph.html:
        Fragment #impl-Index%3CId%3E at struct.EGraph.html does not exist!
        Fragment #impl-IndexMut%3CId%3E at struct.EGraph.html does not exist!