Make WOFF2 brotli decoding optional

[fschutt]

The brotli-decompressor crate has some C dependencies, which is a problem when compiling allsorts to WASM (in order to use in on the web and on serverless-wasm applications). Specifically, some tools have problems with mixed C / Rust dependencies.

This PR makes --feature=brotli optional, but enabled by default.

[wezm]

I'm ok with the overall goal of the PR. However doing some review and testing I'm wondering if this should be a feature to enable/disable WOFF2 support instead of just brotli. It seems that it's not possible to use the ReadBinary impl for Woff2Font without the brotli feature enabled, which in turn makes it unlikely you're going to be able to call the extended_metadata method, which will also fail if the brotli feature is disabled.

Woff2Font being the primary interface to WOFF2 fonts I'm leaning towards a woff2 cargo feature that enables/disables the whole woff2 module. That way there isn't the possibility of building code that will only fail at runtime. For example, if someone used no-default-features and did not add brotli back they could write code that would compile, but then fail when used to read a WOFF2 font. In the alternative where the woff2 module is conditional on the woff2 feature the code will fail to build if the feature was absent.

Does that sound like a reasonable alternative?

[fschutt]

Yeah ok, the reason I did it like this is because I didn't want to break the API.

[wezm]

Did you want to tackle that change or are you wanting me to do it?