Changed lambdastr to use dummy for all symbols if dummify=True (fixes #7249)

Previously, if the user declared alpha = symbols('\alpha'), lambdastr would try to use \alpha as a variable, resulting in a syntax error in lambdify. Now all all symbols are replaced with dummies if dummify is true, so any string is a valid variable name.

If the user wants to not use Dummy symbols, they can still set dummify=False, and get direct substitution. This allows the output to be examined, but will result in a syntax error in lambdify if a non-valid symbol name is used (such as \alpha)