First pass at proptest ValueTree for query condition

[rroelke]

This pull request implements a custom ValueTree for query conditions. This initial pass constructs a ValueTree given a query condition and shrinks the combination clauses.

Without a strategy to generate a query condition, this can only be used to shrink to shrink one that has already identified, e.g.

use strategy_ext::StrategyExt;
Just(my_query_condition).value_tree_map(|vt| QueryConditionValueTree::new(vt.current())

The hope is that this can be useful for customer cases. And someday when we build strategies to generate query conditions it can be useful for shrinking those, too.

[davisp]

+1