Compress ElementId more

[nilsdeppe]

Proposed changes

I hadn't realized you could have IDs smaller than 96 bits with Charm++. I thought it always had to be exactly 96 bits. This saves us another 4 bytes in the ElementId of unused space.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments