This commit refactors trait system for guest types. Namely, as
discussed offline on zulip, `GuestType` now includes `GuestTypeClone`,
whereas `GuestTypeCopy` has been renamed to `GuestTypeTransparent`.
these names were artifacts of some early confusion / bad design i made
in the traits. read and write are much simpler names!
also, change a ptr_mut to ptr where we just read the contents in the
argument marshalling for structs. this has no effect, but it is more
correct.
* atoms in one test unit
* factor out pointers test
* factor structs into separate test unit
* factor out arrays, flags
* finally, separate into strings and ints
