Add name remapping API

[pavel-kirienko]

The idea is roughly like this, borrowed from ROS, not yet finalized:

// Default behavior -- no remapping; assume the namespace is "my/own/ns"
cy_advertise_c(cy, &pub_a,  "foo", 0);  // resolves as "my/own/ns/foo"
cy_advertise_c(cy, &pub_b, "/foo", 0);  // resolves as "foo"

// Introduce remappings:
cy_remap(cy, "/my/own", "/");    // replace prefix "my/own" => "".
cy_remap(cy, "/foo",    "/zoo"); // replace prefix "foo" => "zoo".

// Alternative syntax for the above, same outcome:
cy_remap(cy, "my/own/*", "/");
cy_remap(cy, "foo/*",    "/zoo");

cy_advertise_c(cy, &pub_c,  "foo", 0);  // resolves as "ns/foo"
cy_advertise_c(cy, &pub_d, "/foo", 0);  // resolves as "zoo"

The implementation is absolutely straightforward and compact: we add a new WKV container that keeps the remappings suffixed with the * pattern; in the above case that would be my/own/* and foo/*. The first pattern to match defines the remapping: we take the prefix from the container and join it with the matched substitutions: ("my/own"=>"") / "ns" / "foo", etc.

The same way we could even do much more complex remappings like my/?/ns, although the need for that is not yet evident.