Change C-a and C-e bindings to move to beginning/end of text (ignoring comments).

New behavior is:

- `C-a` or `my/move-start-of-line` will move point to the beginning of
  line as usual. If already there, it will move point to the beginning
  of the text on the line (e.g. after indent, comment chars).

- `C-e` of `my/move-end-of-line` will move point to the end of the
  line as usual. If already there, it will move point to the end of
  the text before any trailing comment.