Heavy ECS - hecs shim

This crate exists to provide a shim between hecs and Heavy, for the reason that at current Heavy depends on several extensions to hecs we hope to upstream soon. Until that happens, this crate contains direct copy of our fork of the hecs source code, subject to the same license as hecs itself.

Once the fork is upstreamed this crate will become a single module with a single line that reexports hecs.

This is not expected to be used outside of Heavy.