Refactor the TypeConverter to support more robust type conversions:

* Support for 1->0 type mappings, i.e. when the argument is being removed.
* Reordering types when converting a type signature.
* Adding new inputs when converting a type signature.

This cl also lays down the initial foundation for supporting 1->N type mappings, but full support will come in a followup.

Moving forward, function signature changes will be driven by populating a SignatureConversion instance. This class contains all of the necessary information for adding/removing/remapping function signatures; e.g. addInputs, addResults, remapInputs, etc.

PiperOrigin-RevId: 254064665