switch from our findvars! to MTK's collect_vars!

[isaacsas]

Since the latter is more likely to be updated as things change in MTK.

[isaacsas]

#1077 gets us started by making it work on Reactions.

[isaacsas]

This is done by #1104 modulo that there is no collect_vars! currently for events. So events still use our findvars!, but note this means they are not handling scoped variables consistent to collect_vars! (since we don't distinguish them in findvars!).