Add support for function-like AST nodes

Substrait has some AST nodes which are (in my opinionated view) scalar functions in disguise:

```
IfThen
SwitchExpression
SingularOrList
MultiOrList
Cast
Nested (maybe... or maybe this shouldn't look like a scalar function)
```

My preference would be to add them into the API so they look no different than other scalar functions such as `add`, etc.