InputMapping does not infer available type mappings on polymorphic operators

[glopesdev]

The InputMapping node is invaluable in simplifying type conversion logic by doing inference over available overloads in the successor node. Currently, this ability does not extend to polymorphic operators, which blocks integration of polymorphic types as first-class citizens in the language.

Support seems relatively simple and non-committal to implement using the ICustomTypeDescriptor interface. Since type mappings are purely based on editor inference, future-proofing is guaranteed at the language model level where types are indeed explicitly declared (i.e. if this type inference approach were to change in the future, nothing in the serialized workflow model would need to change).