replace-buffer-contents cleanups

replace-buffer-contents cleanups

* src/editfns.c (NOTE_DELETE, NOTE_INSERT): Avoid unnecessary parens.
(Freplace_buffer_contents): Check args before returning results.
Avoid integer overflow when computing too_expensive, and work even
if MAX-COSTS is bignum.  Call alloca and/or malloc just once, not
three times.
(set_bit, bit_is_set): Simplify micro-optimization by using eassume.