`return distribution(gen)` is a data race between a read and a write in
two threads, reported by TSan. Remove static random number generators so
they aren't using the same generator.
Removes common_sizes.h in favor of having `_KiB`, `_MiB`, `_GiB`, etc
user-literals within literals.h.
To keep the global namespace clean, users will have to use:
```
using namespace Common::Literals;
```
to access these literals.
