runtime: splitting classes into two files

[alyssawilk]

Risk Level: n/a (just moving code)
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

[alyssawilk]

while trying to resussitate #8847 I ended up trying to call disallowedByDefault a few other places in the code base to verify correctness moving from old to new naming.
Turns out that's really hard to do, because you end up in proto dependency loops due to the proto utilities using disallowedByDefault, but runtime_features.h using proto utilities.
Breaking that loop in master to save myself the pain should it come up again.

[mattklein123]

Nice cleanup.

[sunjayBhatia]

just an FYI, these changes seem to have not agreed with MSVC/Windows somehow, we will likely report back here or on slack with questions and potentially an additional patch cc @wrowe