Implement higher-ranked trait bounds

[brson]

This is on the 1.0 schedule, but there is no issue and no RFC.

[reem]

Will there be an RFC? Is there something we can look at to see what this might look like?

[brson]

@reem @nikomatsakis is working on an RFC.

[brson]

This blocks converting to unboxed closures.

[pnkfelix]

P-backcompat-lang, 1.0.

[ftxqxd]

Isn’t this #15067?

[pcwalton]

@nikomatsakis is starting to work on this.