Add _<bool>("s1", "s2") ternary & use TYPE_CASTER

This commit adds an additional _ template function for compile-time
selection between two description strings.  This in turn allows the
elimination of needing two name() methods in type_caster<arithmetic
types> and type_caster<eigen types>, which allows them to start using
PYBIND11_TYPE_CASTER instead, simplifying their code by eliminating all
the code that they are duplicating from the macro.