By removing the constness of the type for the actual stored value, the
return values of the access operators are defined by the type itself. This
allows both const and non-const types to be stored in a rworker_local
object.
Given that worker.hh was public, it made sense to make routingworker.hh
public as well. This removes the need to include private headers in
modules and allows C++ constructs to be used in C++ code when previously
only the C API was available.
