Move stream related code into a new module.

Stream releated code is shared between both the basic and multi-threaded
interfaces.  I've moved it into a new module.

The fcgx_puts predicate now takes the output stream that the output should
be written to.

streams.m:
mfcgi.m:
README.md:
    Add the new module.

basic.m:
    Remove the streams code from this module.

streams.m:
    The fcgx_puts predicate now takes the output stream that the output
    should be written to.

mfcgi_test.m:
    Conform to the above changes.