Optimize Fields of Spatial Maps

[Chris3606]

Currently the SpatialMap and MultiSpatialMap implement Items and Positions using yield return. Instead, they could just return the Keys and Items fields of the underlying dictionaries. These enumerators are likely to be more efficient.

We could also provide overloads that return the actual type instead of IEnumerable so that we could take advantage of those optimizations when the specific type is known.