Validate that each function used is defined

[jasonkuhrt]

For example:

1. In the spec, each function used, must be defined in the standard library:

{ input: { $whoops: 5 } }

Errors:

(1) Function "$whoops" is not defined.

{
  input: {            
    $whoops: 5
     ^^^^^^ (1)
  }
}

Solutions:

(1) <docs about the standard library>