Make hashbrown optional

[not-fl3]

hashbrown became a fairly heavy dependency:

├── fontdue v0.7.3
│   ├── hashbrown v0.13.2
│   │   └── ahash v0.8.11
│   │       ├── cfg-if v1.0.0
│   │       ├── once_cell v1.19.0
│   │       └── zerocopy v0.7.34
│   │           └── zerocopy-derive v0.7.34 (proc-macro)
│   │               ├── proc-macro2 v1.0.82
│   │               │   └── unicode-ident v1.0.12
│   │               ├── quote v1.0.36
│   │               │   └── proc-macro2 v1.0.82
│   │               │       └── unicode-ident v1.0.12
│   │               └── syn v2.0.63
│   │                   ├── proc-macro2 v1.0.82
│   │                   │   └── unicode-ident v1.0.12
│   │                   ├── quote v1.0.36
│   │                   │   └── proc-macro2 v1.0.82
│   │                   │       └── unicode-ident v1.0.12
│   │                   └── unicode-ident v1.0.12
│   │       [build-dependencies]
│   │       └── version_check v0.9.4
│   └── ttf-parser v0.15.2

hashbrown had the same problem in the past: tkaitchuck/aHash#18), I believe it might be fixed again (I should do a PR there too!).

But so far - it would be nice to just fallback to std.

[mooman219]

ah yeah that's unfortunate, sounds good to make it optional