fontique: Loading fonts from fs

[waywardmonkeys]

We should be able to load a font from the file system or scan a directory for fonts, rather than only having FS access via system. (We should be able to let the text stack memory map arbitrary font files without having to load them into a blob.)

In fontdb, this is easy to do by using Database::load_font_file or Database::load_font_dir.

In font-kit, this can be done with the sources::fs::FsSource.

[waywardmonkeys]

I was wrong about part of the above. Blob works with memmap2 so that you aren't loading the whole thing. But the other stuff is still true, I think.

[dfrg]

Having register_fonts take a Blob instead of Vec<u8> would be one step in this direction. We could add register_fonts_from_path to solve the rest, but we’d want to consider whether these should be fully loaded or just parsed and added to a source path map to be lazily loaded later.

Also, since memory mapping is considered generally unsafe in Rust, we should offer the option to disable it, even for system collections.