The /tg/-style movespeed_modifier datums were not incorporated into the
original smooth movement PR. While
https://github.com/Aurorastation/Aurora.3/pull/22043 restores that
functionality, the original values no longer feel good now that we're on
smooth movement. This PR halves ALL movespeed_modifier/slowdown datums,
with the exception of species-native, as a baseline before more
individual adjustments are made.