Add the unsized keyword

[nrc]

Add an unsized keyword which can be used on traits and trait bounds. It should be feature gated and we might change the keyword later (type has been suggested). We will compute the 'sized'-ness of every type and ensure compatibility of types.

Blocks #12938

Assigned to me.

[flaper87]

@nick29581 I've added this bug to the list of sub-bugs on #12938, we should do the same with the upcoming bugs blocking DST

[brson]

@nick29581 Is this done with 696f16e merged?

[nrc]

Yes, it is (although we are going with a totally different syntax which is still to do)