Found some more unsafe operations in DataFixers.
Also replaced quite a few bad uses of Map.containsKey
containsKey is a common newbie mistake that "reads" cleaner, but
results in double the performance cost of all map operations as
containsKey in MOST cases where null values are not used is identical to get() == null
Considering how deep datafixers go in call stacks, with tons of map lookups,
this micro optimization could provide some gains.
Additionally, many of the containsKey/get/put style operations were
also a concurrency risk, resulting in multiple computation/insertions.
