Auto merge of #8818 - ehuss:weak-dep-features, r=alexcrichton

Auto merge of #8818 - ehuss:weak-dep-features, r=alexcrichton

Implement weak dependency features.

This adds the feature syntax `dep_name?/feat_name` with a `?` to only enable `feat_name` if the optional dependency `dep_name` is enabled through some other means. See `unstable.md` for documentation.

This only works with the new feature resolver. I don't think I understand the dependency resolver well enough to implement it there. It would require teaching it to defer activating a feature, but due to the backtracking nature, I don't really know how to accomplish that. I don't think it matters, the main drawback is that the dependency resolver will be slightly more constrained, but in practice I doubt it will ever matter.

Closes #3494

**Question**
* An alternate syntax I considered was `dep_name?feat_name` (without the slash), what do people think?  For some reason the `?/` seems kinda awkward to me.