Use -Zallow-features=rustc_private

[bjorn3]

I try to avoid unstable features where possible. This option enforces this.

[bjorn3]

Blocked on dtolnay/anyhow#230 (fixed at some point)

[bjorn3]

Blocked on SergioBenitez/version_check#20 (build dependency of ahash)

Edit: Blocked on a new release with rust-lang/hashbrown#357

[bjorn3]

Now blocked on updating indexmap and cranelift to hashbrown 0.13.1.

[bjorn3]

The update_cranelift branch does this. Currently blocked on a release of cranelift 0.100.