We are now freely flowing computed and output properties across the
RPC boundary with providers. As such, we need to tolerate them in
a few more places. Namely, mapping to and from regular non-resource
property values, and also when copying RPC resource state back onto
live runtime objects.
This adds a handy MapReplace function on pkg/resource's PropertyMap and
PropertyValue types. This is just like the existing Mappable function,
except that it permits easy replacement of elements as the map transformation
occurs. We need this to perform float64=>int transformations.
