Treat out-of-range positions consistently

Treat out-of-range positions consistently

If a position argument to get-byte etc. is an out-of-range integer,
treat it the same regardless of whether it is a fixnum or a bignum.
* src/buffer.c (fix_position): New function.
* src/buffer.c (validate_region):
* src/character.c (Fget_byte):
* src/coding.c (Ffind_coding_systems_region_internal)
(Fcheck_coding_systems_region):
* src/composite.c (Ffind_composition_internal):
* src/editfns.c (Fposition_bytes, Fchar_after, Fchar_before)
(Finsert_buffer_substring, Fcompare_buffer_substrings)
(Fnarrow_to_region):
* src/fns.c (Fsecure_hash_algorithms):
* src/font.c (Finternal_char_font, Ffont_at):
* src/fringe.c (Ffringe_bitmaps_at_pos):
* src/search.c (search_command):
* src/textprop.c (get_char_property_and_overlay):
* src/window.c (Fpos_visible_in_window_p):
* src/xdisp.c (Fwindow_text_pixel_size):
Use it instead of CHECK_FIXNUM_COERCE_MARKER, so that
the code is simpler and treats bignums consistently with fixnums.
* src/buffer.h (CHECK_FIXNUM_COERCE_MARKER): Define here
rather than in lisp.h, and reimplement in terms of fix_position
so that it treats bignums consistently with fixnums.
* src/lisp.h (CHECK_FIXNUM_COERCE_MARKER): Move to buffer.h.
* src/textprop.c (validate_interval_range): Signal with original
bounds rather than modified ones.