Asynchronous insertion for dwarf-mode.el

Asynchronous insertion for dwarf-mode.el

I was recently examining a very large .debug file.  I tried to use
dwarf-mode, but it blocked Emacs for a very long time while reading
output.

This patch changes dwarf-mode to run the objdump process asynchronously.
This way, I can still do other things in Emacs while waiting for the
dumping to finish.

2017-10-10  Tom Tromey  <tom@tromey.com>

	* dwarf-mode.el (dwarf--process, dwarf--deletion-region): New
	defvar.
	(dwarf--check-running, dwarf--sentinel, dwarf--invoke)
	(dwarf--filter): New functions.
	(dwarf-do-insert-substructure, dwarf-do-refresh): Call
	dwarf--check-running, dwarf--invoke.
	(dwarf-browse): Initialize new variables.