factor out the transport-from-env bit
correctly name the class invocant vars
make room for alternate env var prefix
factor out "easy transport building" for use in Wrapper
Wrapper should proxy the question of simpleness
allow a Wrapper transport to be configured by env
v1.300003

          make Wrapper transports proxy is_simple and allow_partial_success to
          the wrapped transport

          allow Wrapper transports to construct their own targets; useful for
          configuring entire wrapper chain from the environment