Replace ParseFuncOrArgument with type-safer alternative. (#901)
Replace ParseFuncOrArgument with type-safer alternative. (#901)

* Replace ParseFuncOrArgument with type-safer alternative.

In the process, document consequences of implementation
details that make type-safety difficult to implement.

* Parser: Added assertFuncOrArg to ensure type-safety.

* Responded to comments.

* Made token mandatory for ParsedFuncOrArgOrDollar.

azotova/KaTeX,Khan/KaTeX,xymostech/KaTeX,azotova/KaTeX,kwangkim/KaTeX,kwangkim/KaTeX,kwangkim/KaTeX,xymostech/KaTeX,kwangkim/KaTeX,kwangkim/KaTeX,xymostech/KaTeX,Khan/KaTeX,azotova/KaTeX,azotova/KaTeX,azotova/KaTeX,xymostech/KaTeX,Khan/KaTeX,xymostech/KaTeX