emacs: nice bindings for common navigation cmds

previous-error, next-error, and goto-line are all very commonly used
commands with quite awkward default keybindings. by default they are in
the "M-g" prefix: "M-g M-p", "M-g M-n", and "M-g M-g". This does funny
things to my hands, especially when jumping through grep matches, so
drop the "M-g" prefix.

"M-p" is now previous-error.
"M-n" is now next-error.
"M-g" is now goto-line.

Some modes will override these, but probably not where I'll want to use
them anyway.