Make special case differential operator

[runarberg]

When an inputted d is most likely a part of a differential
automatically map it to an italic operator with appropriate
spacing.

Also fix spacing issues around operators inside fractions and
groups.

Output Changes

• Differential d now renders as an operator with and rspace of 0. If
  inside fractions it also has an lspace of 0.
• First operators in groups now have an lspace of 0. This was true of
  all first operators in matricies. But now it applies to all
  operators in groups, unless it is a lone operator, in which case it
  has it by default.
• Spaces after a singleton group element no longer puts the element in
  an unessisary .

Resolves: #53