2229: Capture box completely in move closures
Even if the content from box is used in a sharef-ref context,
we capture the box entirerly.
This is motivated by:
1) We only capture data that is on the stack.
2) Capturing data from within the box might end up moving more data than
the user anticipated.
Closes https://github.com/rust-lang/project-rfc-2229/issues/50
r? `@nikomatsakis`
a4f832b275