Cleanup of ugly MI and SOL shaped noteheads

1. Reducing width of MI notehead, for both "regular" (Aiken, etc.)
   and "small" (Funk/Walker) heads. When Funk/Walker heads were
   introduced, a special width variable was introduced to both
   sets and the mi heads were made wider than the other noteheads.
   This does not seem to be the practice of available examples
   (Sacred Harp, Southern Harmony books, etc.) and causes issues
   with alignment, particularly with stems up, and is visually odd.
   The variables have been removed from the metafont code, making
   the MI noteheads the same width as the other shaped notes in
   their respective sets.

2. Changing the SOL notehead to more closely match the normal round
   notehead (as advertised in the mf code). The previous code uses
   the shape of the open (half note) notehead, which is visually
   unappealing. The open notehead has been defined so that it
   matches visually with both the normal Aiken/Sacred Harp heads,
   as well as the thin variants (in keeping with observed practice,
   which uses the regular open roundhead even when thin variants
   are used).