Issue 273b

[jverzani]

#281 was one way to address #273, this is another:

• uses different dispatch for real, imag (Like N which is more performant)
• adds a few types for dispatch in that manner
• Deprecated real, imag, and conj for expressions with free symbols

Issue #273 points out that symbolic variables are subtypes of Number, not Real, so the definition of real(x) = x isn't correct. That seems right. Since there isn't any underlying support for this functionality in libsymengine there isn't a means to define an unevaluated real(x) or imag(x). So it seems like an error is appropriate. (Already imag(x) was erroring, though with an incorrect call to InexactError, likely my mistake awhile ago.) As this change would be breaking, this PR sets up a deprecation and adds suggested code (commented out) for when the breaking release is ready.

If #292 gets merged first, there are few instances of Basic(0) that could be replaced by ZERO and a few constants to be added to the enumeration in RealConstants. So I'd update this PR.