Add VSA set types

[twizmwazin]

The idea is basically to make VSA set types explicit and remove set stuff polluting BV. Further, let's implement all of the operations as simplifiers instead of a backend. This will allow expressions to be automatically simplified without invoking a backend or solver, with the usual annotation preservation logic.

Later, I will update the VSA backend to be basically nothing, since the only thing it would be useful for is running simplifiers without annotation preservation when calling eval.