Support ValueSet as a first-class type

(Updated description in Oct 2022 during backlog cleanup)

The CQL ValueSet type represents a reference to a value set, allowing that value set to be passed into CQL functions without the need to do a value set expansion (as in CQL 1.4).

To demonstrate how this might work, consider the following example:

library MyLibrary version '0.0.1'
valueset MyVS: 'http://example.org/vs/myvs'
codesystem MyCS: 'http://example.org/cs/mycs'
code Foo: 'Foo' from "MyCS" display 'Foo'

define function FooInVS(vs ValueSet):
  Foo in vs

define IsFooInMyVS:
  FooInVS(MyVS)

This results in the following JSON:

[
  {
    "name": "FooInVS",
    "context": "Patient",
    "accessLevel": "Public",
    "type": "FunctionDef",
    "expression": {
      "type": "InValueSet",
      "code": {
        "name": "Foo",
        "type": "CodeRef"
      },
      "valueset": {
        "name": "vs",
        "type": "OperandRef"
      }
    },
    "operand": [
      {
        "name": "vs",
        "operandTypeSpecifier": {
          "name": "{urn:hl7-org:elm-types:r1}ValueSet",
          "type": "NamedTypeSpecifier"
        }
      }
    ]
  },
  {
    "name": "IsFooInMyVS",
    "context": "Patient",
    "accessLevel": "Public",
    "expression": {
      "name": "FooInVS",
      "type": "FunctionRef",
      "operand": [
        {
          "name": "MyVS",
          "type": "ValueSetRef"
        }
      ]
    }
  }
]

Note that the function indicates that it takes a ValueSet, but when you invoke the function, it's actually a ValueSetRef that is passed in.

Initial Zulip conversation: https://chat.fhir.org/#narrow/stream/179220-cql/topic/ValueSet.20Typing

Start with the CQL ValueSet example in the issue and read the linked CQL reference plus the initial Zulip conversation to clarify the intended typing semantics. Trace how function parameters and ValueSetRef arguments are represented and executed; done means the example supports a first-class ValueSet parameter without requiring value set expansion.