Rollup merge of #49179 - varkor:future-deprecation, r=QuietMisdreavus,GuillaumeGomez

Rollup merge of #49179 - varkor:future-deprecation, r=QuietMisdreavus,GuillaumeGomez

Handle future deprecation annotations

This adds special handling to the `since` parameter of the `deprecated` attribute: in particular, if the `since` version exceeds the version of the compiler, the deprecation notice will not be printed; but a note is added to the documentation stating that the item will be deprecated in a later version.

(I've used `since` for this, rather than adding a new attribute, because it's more seamless and, I feel, intuitive. Plus it involves less code churn.)

![image](https://user-images.githubusercontent.com/3943692/37611317-ef5cdf16-2b99-11e8-8251-e35e8f7b0137.png)
![image](https://user-images.githubusercontent.com/3943692/37611323-f748c2d0-2b99-11e8-966b-11408c73d416.png)

This is a prerequisite for doing things renaming methods in the standard library (e.g. #30459). Resolves #30785.