Fix Chain::RemovePrefixSlow(): if the block pointer array is inline then begin_ must remain unchanged.
Fix Chain::RemovePrefixSlow(): if the block pointer array is inline then
begin_ must remain unchanged.

Cleanup in Chain::ReserveFrontSlow(): this function is no longer called
with the argument of 0, so the code which shifts block pointers can be
simplified.

Make comments more uniform.

PiperOrigin-RevId: 196379252

google/riegeli,google/riegeli,google/riegeli,google/riegeli