Add top-level Cargo.toml workspace definition per zome

[maackle]

Whenever running hc generate, I get this warning. It seems like it would be a good idea to generate a top-level Cargo.toml to create a workspace, which will not only squelch this error but help zomes work better with IDEs.

hc g zomes/hosts
> cargo init --lib --vcs none
warning: compiling this new crate may not work due to invalid workspace configuration

current package believes it's in a workspace when it's not:
current:   /Users/michael/Holo/hosting-app/zomes/hosts/code/./Cargo.toml
workspace: /Users/michael/Holo/hosting-app/Cargo.toml

this may be fixable by adding `zomes/hosts/code` to the `workspace.members` array of the manifest located at: /Users/michael/Holo/hosting-app/Cargo.toml
     Created library package

[maackle]

Note, if we do this, we need to update the .build configs because the built artifacts will wind up not in .target/ but in the root workspace's target dir