Code Equivalence needs to be updated based on Errata

[Bryn Rhodes]

Code equivalence was relaxed in an errata update to CQL to ignore version, but the engine is still considering it for equivalence:

define C1: 
  Code { 
    code: 'ABC',
    display: 'Code ABC',
    system: 'http://example.com',
    version: '2017-01'
  }

define C2:
  Code {
    code: 'ABC',
    display: 'Variant Description',
    system: 'http://example.com',
    version: '2017-05'
  }

define CEqual: C1 = C2 // false
define CEquivalent: C1 ~ C2 // true

[c-schuler]

Addressed in #93