By default, when compiling more than one file, a shared libraryis generated with a file derived from the sha of compiled modules(i.e. the group name). This is also used in the names of generated.c and .h files. Add experimental `group_name` argument to `mypycify`that allows overriding this. This can be useful when integratingmypyc to a build system, as this makes the names of output filesmore predictable.