add some predicates; coeff

[jverzani]

• Adds some generic predicates: isreal, isinteger
• useis_a_Symbol for is_symbol
• add is_symbol check in
• adds internal functions to expose several is_a_XXX functions (e.g. is_a_Number, is_a_Integer, ...)
• exposes basic_coeff as unexported coeff function

This started out by trying to find a zero allocation check for whether an object is a symbol and for has_symbol. But along the way, realized some other useful pieces of cwrapper might be exposed.