Add SharedPort wrapper around rt::comm::SharedPort

SharedPort implementation was missing in std::comm. Since this module
also wraps SharedChan, it makes sense to have SharedPort defined there
as well.
auto merge of #9198 : FlaPer87/rust/shared-port, r=cmr

SharedPort implementation was missing in std::comm. Since this module
also wraps SharedChan, it makes sense to have SharedPort defined there
as well.