Evaluate hex encoding for embedding files?

It was just pointed out to me [in a Hacker News thread](https://news.ycombinator.com/item?id=18113129) that hex might be more efficient than base64 after gzip/brotli compression for `SINGLE_FILE`'s embedding.

His test showed that a hex-encoded wasm file was 65% the size of a base64-encoded wasm file after compression. My own test with a random 4.7 MB png I had sitting around showed that it didn't make a difference — with brotli they were all about the same (hex having the edge over base64 by 215 B and over the compressed original file by ~2.5 KB) and gzip was similar (in this case hex was ~600 KB bigger) — so it's unclear what the impact would be.

Just calling attention to this on the off chance that it would actually be an improvement, as I don't recall hex being considered when originally writing the `SINGLE_FILE` PR, and the previous discussion on encoding efficiencies (https://github.com/kripken/emscripten/pull/3326#issuecomment-91352434) has no mention of hex.