Support connections between Slots whose types are implicitly convertible

I'm not sure how complicated this is to implement (if possible at all), but it would be great two slots could be connected if their types are convertible, e.g.:
```
Input myIVec4Input;
Output myVec4Output;

myIVec4Input << myVec4Output;
```
Currently, this gives a compile error: `binary '<<': no operator found which takes a right-hand operand of type 'gloperate::Input' (or there is no acceptable conversion)`

Start by locating the Input and Output slot types and the operator used by `myIVec4Input << myVec4Output`. Read how slot types are checked during connection, then determine the intended behavior for the shown glm::ivec4-to-glm::vec4 example and verify that convertible slots can connect.

Written by the indexing model from the issue text.