Rollup merge of rust-lang#63855 - killercup:refactor/feature-gates, r…

…=Centril

Refactor feature gates

After rust-lang#63824, this goes a few steps further by

- parsing doc comments in the macros to extract descriptions for feature gates, and
- introducing a common `Feature` type to replace the tuples used previously to improve readability.

The descriptions are not yet used, but I felt like this PR is a useful enough refactoring on its own.

r? @Centril