Function calls about arcaflow-expressions

[janosdebugs]

@jaredoconnell another thought: the ArcaflowFunctionSchema declares its input schema. However, I think it would be better to change this to an interface, which looks like this:

type ArcaflowFunctionSchema interface {
    // InputSchemaHint gives one or more valid input types the function can accept. This is for documentation purposes only, but when run against CanAcceptInput or OutputSchema, must result in a valid result.
    InputSchemaHints() []schema.Type
    // CanAcceptInput lets a function determine if it can accept a certain input.
    CanAcceptInput(schema.Type) bool
    // OutputSchema produces an output schema given an input schema.
    OutputSchema(schema.Type) (schema.Type, error)
}

Accordingly, we may change the ArcaflowFunction to not have generic types and accept an any as input and produce an any as output. This will make functions more flexible.

[dustinblack]

@jaredoconnell Isn't this closed with #16 ?

[jaredoconnell]

Yes. This is out of sync with JIRA.