[pxl-build] Add build-time resource loader

- [ ] binary blobs
- [ ] update quote, syn, proc-macro2, etc
- [ ] glsl shaders (will be precompiled?)
- [ ] images
- [ ] audio
- [ ] text
- [ ] format generated code with rustfmt if available
- [ ] add readme and document extensions
- [ ] bitmap fonts
- [ ] multiple sizes
- [ ] font metrics
- [ ] ttf fonts
- [ ] messagepack encoded objects
- [ ] json encoded objects

Using large images with `pxl-build` is prohibitively slow. It should be fast, even with enormous images. I think I'll need to generate binary blobs with the build function, and access them with `include_bytes!()` or extern symbols that are resolved by the linker.

Additionally, it is not tenable to store images and uncompressed RGBA 32 bit floating point. I'll need to define an image class that has the ability to convert between various formats to a display pixel.

Start at the pxl-build build function and review the proposed include_bytes! or extern-symbol approach for generated binary blobs. Use the checklist to scope supported resources, image format conversion, rustfmt handling, and README documentation; done would require an agreed implementation covering these requirements.

Written by the indexing model from the issue text.