Add a "resource bundle" for files like images, fonts, and localization

Nobody has claimed this yet.

This came up in the #353 discussion. I'll let @cmyr explain it:

> My long-term vision for this is that you shouldn't be loading data directly from the file system; there should be a 'resource bundle' that druid exposes that contains various types of resources (image files, font files, localization files) and that is validated at compile time, so that at runtime getting a resource looks something like,

```rust
let my_svg: SvgData = env.get_bundle_resource(MY_SVG_IDENTIFIER);
```

> or maybe even in a very fancy world there's a build phase that generates per-app marker 'keys' (like env::Key) for each of the resources in this app's bundle?

Start by reading the #353 discussion and the proposed env::Key and get_bundle_resource entry points described here. Before implementation, clarify the resource types, compile-time validation, bundle format, and acceptance criteria; the work is done only when those decisions are reflected in a usable resource-bundle API.

Written by the indexing model from the issue text.