Update for Rust 1.57

[kornelski]

Rust 1.57 has stabilized the try_reserve feature.

This change:

• Uses "native" try_reserve if rust_1_57 feature is enabled. I haven't added auto-detection, because it'd require a build script.
• Keeps using hashbrown::TryReserveError, because it's already a part of the stable API.
• Adds a std feature, because HashMap isn't in the alloc crate.
• Supports a combination of unstable and rust_1_57 to correct for changes in the more recent nightly Rust versions.

I think this change is backwards compatible, and keeps the same MSRV.