`rustc-hash` feature is non-additive.

cargo features are supposed to be additive, but rustc-hash isn't.

Consider the case where:

1. Crate a depends on rustdoc-types with default features
   • Crate a relies on the HashMap using the default hasher.
2. Crate b depends on rustdoc-types with rustc-hash feature.
3. Crate c depends on a and b

• Cargo does feature unification, and builds a agains rustdoc-types with rustc-hash enabled.
• a fails to compile.

Potential fixes

1. Ignore this, and hope it doesn't effect anyone
2. Implement our own BuildHasher type that doesn't expose the underlying hashing algo, so changing it is additive
3. Drop the rustc-hash feature, and always use it.
   3.1. Drop the rustc-hash feature, and never use it.
4. Make Crate generic over the hasher type.

I think it's fine to go with 1 for now, but I'm filling this issue so I don't forget this.

Originally reported by @noratrieb on fediverse

Caused by #42

The issue concerns the rustc-hash Cargo feature and Cargo's feature unification, but it names no source files or tests. Start by tracing how the rustc-hash feature changes the HashMap hasher in rustdoc-types. Done requires choosing and implementing one of the listed approaches while preserving additive feature behavior.