feat(isthmus): add up-converting signature matchers, and coerce types to match

[bvolpato]

My attempt to take a stab at the TODO in the code:

  // TODO: define up-converting matchers.

We've got here since we noticed that functions that have list<any1>, any1 arguments wouldn't match list<string>, string without implementing a proper CallConverter as adapter:

• The FunctionConverter was not matching using directMatchKey since list_any != list_str.

• When falling back to the leastRestrictive logic, list<string> + string would also not agree, so no functions were returned.

I've kept the leastRestrictive logic intact, but added another fallback matcher that checks if all arguments will match (using the existent isMatch logic, which relies on IgnoreNullableAndParameters internally).

--

Let me know if there are any concerns or thoughts on how to make this better.

[vbarua]

Started taking a look and left some minor feedback. I can look at this some more tomorrow. I need to be fresher to really grok the type matching code 🧠

[vbarua]

Left some more comments.

[vbarua]

Thanks for adding this in!