Converts reagents to use typepaths instead of IDs.
Centralises reagent spawning in a lot of places to use reagents_to_add instead of a variety of custom methods or initialise overrides.
This PR will lead us towards the Promised Day, for in its wake there shall be much celebration and ecstasy as this world becomes a world suitable for developer hegemony. The first strike is thusly;
All << is converted into to_chat().
Fixes lag from plants processing forever instead of obeying the subsystem's tick interval. Also fixes a glitch where plants didn't continue to grow if not mature.
Fixes#3451.