Misc tweaks

[kornelski]

• Modules don't need to exist as (tiny) files.
• Enums without a non-exhaustive attribute may as well be changed to be copyable.
• to_vec copies, but into can reuse existing allocation.
• It'd be nice to change &self to self on execute to avoid copying in other cases.

[jmgilman]

Thanks for the PR, looks like some tests are failing. I can try to look into this more in-depth next week.

[kornelski]

Yeah, these tests print "wip" for me, so I assume that's something unrelated unfinished.