Bevy needs a simpler way to load in thousands of icon textures as an atlas

## What problem does this solve or what need does it fill?
I am trying to add 500+ icons (64x/64 px) into my game. However, using bevy_asset_loader it seems like it takes a massive amount of memory.

## What solution would you like?

I would like to be able to more efficiently pack my textures into an atlas and load them as a serialized atlas . However there is no such thing--- serialized atlas in 0.14 includes handle ids which wont work ... need a table of string -> coords and dimensions !

## What alternative(s) have you considered?

## Additional context