Patterns for constant and symbolic terms

[HarrisonGrodin]

It would be helpful to be able to represent via a pattern that certain sub-terms may only be constants or instances of Symbolics. These assertions would likely appear in Variables (or as a new type of variable).

• Constant patterns would eliminate the need for the specialized DiffRule (and allow the input pattern of EvalRule to be described precisely).
• Symbolic patterns may be useful as a means of specifying that an argument must be a symbol in order for a term to make logical sense (e.g. index variable for sums/products, infinitesimal terms dx in derivatives/integrals, etc.).