(...and ASSERT_NOT_REACHED => VERIFY_NOT_REACHED)
Since all of these checks are done in release builds as well,
let's rename them to VERIFY to prevent confusion, as everyone is
used to assertions being compiled out in release.
We can introduce a new ASSERT macro that is specifically for debug
checks, but I'm doing this wholesale conversion first since we've
accumulated thousands of these already, and it's not immediately
obvious which ones are suitable for ASSERT.
Implemented move_to_beginning_of_next(), move_to_end_of_next(),
move_to_beginning_of_previous() and move_to_end_of_previous() functions
for more correct word jumping than the move_to_xxx_span() methods that
were previously used.
