Introducing BoxShape

[fasterthanlime]

Something with Ptr and a &'static Shape - we know how to drop it and a whole lot of other things.

Can be useful as an error returned by a parse function, etc.

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Or make it CowShape<'a> so that it can be borrowed, too?

[fasterthanlime]

Or call it Ox ?

[fasterthanlime]

Doing that in #1148