Add sync.Pool to Context for Map and Slice

[xdg]

With a sync.Pool in the Context, generators can use them to get new Maps and Slices, and then users can put them back in the pool via the context for reuse:

obj := factory(context)
// do stuff with obj
context.PutMap(obj)

[xdg]

Would need to recursively walk Map/Slice looking for other Map/Slices to recycle.