Unify the no_std and std impls of FontSystem

[notgull]

This would make it easier to add certain features (i.e. configurable font fallback) to both instances of the FontSystem.

This adds hashbrown and rustc-hash as dependencies for no_std.

[notgull]

@jackpot51 If you have time can you review this change?

[grovesNL]

@notgull won't these dependencies always be enabled on std? I thought they'd be gated under no_std

[notgull]

They do, but they're generally pretty lightweight dependencies.

[grovesNL]

Yeah that's fair, I'm using std but intentionally try to avoid hashbrown when targeting wasm for size, so it's not ideal to include them by default.

[notgull]

That's fair, but I'm not aware of a way to do this in stable Rust.

[grovesNL]

What prevents us from gating hashbrown and a hashmap alias to hashbrown under no_std? I'd be happy to submit a PR to adjust this but not sure if there's something I'm missing.

Edit: opened #168