edit_distance: Rename ED_STACK_ELEMS ED_STACK_DIST_VALS

edit_distance: Rename ED_STACK_ELEMS ED_STACK_DIST_VALS

The previous name was misleading, since it does not define the number of
elements (ed_elem) on the stack, but rather the number of distance
values (ed_dist).  Rename to make this more clear and add more
documentation about what it does and how best to define it.

Note:  This is an API change for custom-compiled versions, but since the
module has only been included for a couple days I don't think it's worth
a back-compat #ifdef at this point.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>