Groups extension for 3.0 #231

The groups extension from #211 (2.1.0) was left out of 3.0.x in the upmerge (#230) and needs to be rebuilt.

In 2.1 the GroupsMiddleware filters the ClassMetadata and passes it on in the stack. With the mapper chain in 3.0 a middleware only gets Mapper $next, so the metadata can't be changed for the mapper anymore. Removing the excluded fields from the extracted data afterwards doesn't work either: the mapper still reads every property, so groups can no longer break circular references (as documented in docs/groups.md) and uninitialized properties fail.

So groups probably have to be supported by the mappers themselves, the ReflectionMapper and the generated mappers. The 2.1 implementation, tests and docs can be used as a starting point.