Fix indentation for list elements on continuation lines

They should line up with the first element on the list, not the first
comma (-:
Improve elixir-debug output

* Make mnemonics more memorable
* Correct them to correspond to what they denote.
Add test case for fn body indentation breakage.
Highlight variables only on assignment

This changes the font lock regex to highlight only when there's a
single = sign in play, not multiple ones (as for comparisons).