Add save/load system

Allow programs to save and load their state. Saves will be managed by the user outside of the program.

Saves may have an optional parent, which indicates that the save is in some way derived from the parent save, for example the parent save may be a prior game state, or a previous document state.

I'm not sure what the UI should be, but program designers shouldn't have to worry about managing saves, they should only worry about loading game state when presented with a save, and creating a save from the current game state when asked to.

This isn't set in stone, but my thinking is to use [msgpack](https://msgpack.org) as the save format, and games can use serde to save and load to msgpack values, which will then be serialized to disk by the runtime.

Games should be saved in a location appropriate to the system, for exmaple `~/Library/Application Support/pxl/saves/NAME/TIME-PARENT.msgpack` on on macOS.

Also, we should use the crate name the key for a game's saves, so that we are guaranteed to avoid collisions as long as the game is published to crates.io.

No source files, tests, or entry points are identified in the issue. Start by locating the runtime and existing platform-specific storage abstractions, then resolve the save UI and API design around serde/msgpack, optional parents, crate-name keys, and system-appropriate paths; done means games can create and load saves reliably.

Written by the indexing model from the issue text.