1 - Use getOrPut to avoid making 2 map lookups where possible.
2 - Use an arena allocator for Values, which makes memory management simpler.
3 - Because of #2, we no longer need to allocate key or values which don't need
to be unescaped. The downside is that the input string has to outlive the
query.Values (but I think this is currently always the case)
4 - Optimize unescape logic & allocations
5 - Improve test coverage
Karl Seguin
94531cb3d0