Added cache_access feature gate and feature docs via document-features crate

[virtualritz]

This adds:

• A cache_access feature gate (off by default).
• Feature documentation via the document-features crate (extracted from Cargo.toml during a doc build).
• A flag to have docs.rs documentation be built with all features enabled.

Side effects:

Both benches and tests now only can be built if the --feature cache_access/--all-features flag is specified.

When I ran cargo bench there were several speed regressions. I dunno if they're circumstantial (from my system). Just though I mention this.