Support container conversions (and maybe other container types?)

[joshlf]

For two possible approaches, see #1183 and #1735.

Currently, we support T -> U conversions using the transmute! macro when T: AsBytes, U: FromBytes, and T and U have the same size. In principle, we ought to be able to support a range of conversions for container types wrapping T and U so long as those containers have well-defined layouts. Specifically:

• Slices
• Box (the docs guarantee Box's layout for Sized types)
• ManuallyDrop (the docs guarantee that ManuallyDrop<T>'s layout is equivalent to T's)
• Wrapping (the docs guarantee that Wrapping<T>'s layout is equivalent to T's)

...and possibly others.

This will help unblock ICU4X using zerocopy.

[ileixe]

To achieve #149, I'd like to try to implement Box for this issue if possible. Could you please give me a guide for it?

[joshlf]

Hey @ileixe , apologies for the radio silence! I likely won't have time to get to this anytime soon unfortunately.

[ileixe]

Hey joshlf, nevermind, we have a life. :) I got an another approach so you don't have to be sorry. Thanks for answer.