setup.py: Separate description for tools and runtime

[abitrolly]

Another non-code fix.

I am not sure if the runtime is not included in tools package. Looks like it is packed as data, but not probably not installed as something to be imported from Python code. I am also not sure why runtime is packed for PyPI if it is only usable by generated Go code.

[alanjds]

You are mostly right. runtime is packed for PyPI because I wanted to let an easy way for people to install Grumpy. As Python is needed anyway, distribution using PyPI seemed as a nice idea. Also because I wanted not to mess with the Golang distribution tools.

grumpy-tools can transpile Python -> Go code
grumpy-runtime have the native modules that the transpilled ones need.

[abitrolly]

This runtime might need some explanation for the users of Python, but for now I couldn't think of anything better. Are there any chance to update PyPI packages?