Removes unnecessary header dependencies that have accumulated over time
as changes have been made. Lessens the amount of files that need to be
rebuilt when the headers change.
While initially done to potentially prevent creating bugs due to C++
having a silly type-promotion mechanism involving types < sizeof(int)
and unsignedness, given that the bulk of these functions' usages
are on exit paths, these can return the correct type to avoid the need
to cast at every usage point.
Due to promotion rules (types < int, even if unsigned, get promoted to
int when arithmetic is performed on them), this is a potential spot for
undefined behavior.
