Use optimal StringComparison for symbol strings

A few searches for symbols ("/", "*", "~/", etc) in strings were using a case insensitive comparison when they can get better performance from an ordinal compare.

- The comparison type used against string.empty is not important but it reads better if it is the same as the rest of that group.
- The LastIndexOf call is using the default comparison which is CurrentCulture. This can be made into a char search which is Ordinal.