Wrap SymEngine::ascii_art() and replace the current function

[abinashmeher999]

Refer symengine/symengine#797 and wrap it to replace the code here. It will also involve writing the C wrapper for the function.

[codeworm96]

Now in the C++ library, the ascii_art returns a string instead of printing it out. Should the ruby wrapper modify its behavior accordingly?

[certik]

@codeworm96 probably. The user can easily print by hand.