on-the-fly switch between port and nif

[zmstone]

we need a way to switch from port to nif or from nif to port.
the switch has to be on-the-fly, at least should not require a beam restart.

[kjellwinblad]

This already exists:

jq:set_implementation_module(jq_nif).
jq:set_implementation_module(jq_port).

But this should be documented in the README file so I leave this open until we have added a description there.

[kjellwinblad]

This is now documented in the README file with the following sentence:

One can use the function jq:set_implementation_module/2 to change the implementation while the application is running.