Proposal: Make rosrust_codegen::genmsg APIs public

[ssnover]

While I've managed to get messages generating using the existing macro by depending on rosrust_msg, I'd like a little finer control over the generation of the package. Currently the rosrust_codegen package does not make any of its APIs public other than the proc macro.

I'd like to have additional APIs made public, particularly from the genmsg module, in order to build a CLI tool which can generate a Rust source file from a ROS package which defines msg and srv files. This would allow me to generate a crate ros_msgs which contained modules for std_msgs, geometry_msgs, etc and then have a lib.rs file which re-exported them. This has the advantage of making the generated source available to read which can be helpful when debugging or when attempting to understand how to use the objects.

I'd like to get such a tool into this repo as well if that seems like an appropriate addition to you.