Make fontc functional for wasm users

[simoncozens]

fontc uses fontbe which uses fea-rs which has a structure Source which stores feature file paths as OsStrings. fea-rs uses the serde feature to serialize these paths for fontbe to do persistable job orchestration.

However, serde does not implement Serialize and Deserialise for OsStrings on the WASM platform, only Unix and Windows, so the whole thing falls down. We could replace it all with Strings to work everywhere.

Additionally, fontbe/src/feature.rs has the concept of an InMemoryResolver which sounds perfect for things which don't have filesystems! Except that it also stores content_path: OsString, which makes it dependent on filesystems again.

[davelab6]

This may be a blocker for using fontc within fontraweb; @rsheeter how important does this sound to you

[simoncozens]

In general: attempting to build fontc and its dependent libraries with --target wasm32-unknown-unknown from time to time is a great way to shake out unintended assumptions.

[cmyr]

Yea, we could definitely get rid of OsStr; the basic idea is that in general we're dealing with paths, but we wanted something that would also work for arbitrary identifiers for the in-memory case, and so I went with OsString. This is easy enough to fix though, if it's a problem!

[rsheeter]

This may be a blocker for using fontc within fontraweb; @rsheeter how important does this sound to you

"may be" or definitely is? - I'd have thought definitely is.

I think it's worth doing but lower priority than making the brand fonts identical on https://googlefonts.github.io/fontc_crater/. If that sounds wrong please lmk.

attempting to build fontc and its dependent libraries with --target wasm32-unknown-unknown from time to time is a great way to shake out unintended assumptions

+1 to this, lets systematically work our way through making sure that works and adding it to CI. I imagine we'd want to start with the fontations libraries. I will rename this issue to reflect the more general problem.

EDIT: apparently we had this idea already for fontations, https://github.com/googlefonts/fontations/blob/main/.github/workflows/rust.yml#L125

[simoncozens]

Adding CI to fontations is a good idea, but I don't anticipate any problems there as we are using read-fonts/write-fonts/skrifa heavily already in Fontspector web (yes it still has Fontbakery branding but it's all Rust underneath).

[rsheeter]

As of #1239 cargo build -p fontc --target wasm32-unknown-unknown succeeds and has been added to CI to ensure it remains so. I have not verified anything actually works in wasm. Baby steps :)