Blog post announcing Rust 2021 feature set

[nikomatsakis]

We plan a blog post announcing the Rust 2021 Feature Set and introducing the key ideas of the edition. This issue is a placeholder to track that work item on the Rust 2021 Project Board.

[nikomatsakis]

@rustbot assign @m-ou-se

[m-ou-se]

rust-lang/blog.rust-lang.org#825