(hexl-mode-old-write-contents-hooks): Remove.

(hexl-mode-old-write-contents-hooks): Remove.
(hexl-mode, hexl-current-address, hexl-address-to-marker)
(hexl-insert-char): Don't hardcode point-min == 1.
(hexl-isearch-search-function): New fun.
(hexl-mode-old-isearch-search-fun-function): New var.
(hexl-mode): Use them.
(hexl-mode, hexl-mode-exit, hexl-maybe-dehexlify-buffer):
Use write-contents-functions rather then write-contents-hooks.