Migration path towards /zoneless #593

Should the /zoneless subpackage become the default.
If so, how should we do it without creating too much hassle to existing codebases?

Proposal:

• re-export the current library as a /zone(?) sub package => feature release
• provide a schematic to update existing import paths to /zone and flag the zone sub package as deprecated => feature release
• make /zoneless the default (with a migration schematic) and remove the /zoneless sub package => major change