Introduce `bitmap256.cc` for `FindNextSetBit()`.

Defining the member function in `bitmap256.h` seems to be
problematic when using Clang modules AKA header modules.

Change-Id: I7f6b1b3aba8739678a0356590a86ded36a18185b
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60930
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Perry Lorier <perryl@google.com>