This patch adds a module that contains both scene and ptcl types (very
lightly adapted from piet-metal), as well as infrastructure for encoding
Rust-side.
WIP, it's not wired up in either the shader or on the Rust side.
This version will be similar but somewhat different to what's in
piet-metal now. Hopefully a little simpler to use (no distinction
between packed and unpacked structs) and better structured.