* Update Caller interface and its documentation.
* Rework MapToRequest as ParamsToRequest.
The old interface returned the result as well as populating it. Nothing was
using this, so drop the duplicated value from the return signature. Clarify the
documentation on the Caller type.
Rework the MapToRequest helper to take an arbitrary value instead of only a
map. This is groundwork for getting rid of the custom marshaling code. For now,
however, the implementation preserves the existing behaviour for the map, until
we can replace those.