Pervasive use of OsString causes problems for wasm

[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.