Refactor operator resolution

[piotrszul]

Encapsulate operator resolution in the manner similar to function resolution.
Decide when do perform the operator resolution (maybe should be at execution time same as for functions)
or maybe the functions should be resolved at parse time as well.

See: Visitor:

  // TODO: encapsulate in operator resolver
  // Decide when do perform the operator resolution (maybe should be at execution time same as for functions)
  // or maybe the functions should be resolved at parse time as well.
  private static final Map<String, BinaryOperator> BINARY_OPERATORS = MethodDefinedOperator.mapOf(
      CollectionOperations.class);

  @Nonnull
  private FhirPath visitBinaryOperator(
      @Nullable final ParseTree leftContext,
      @Nullable final ParseTree rightContext, @Nullable final String operatorName) {
    requireNonNull(operatorName);
    return new EvalOperator(new Visitor().visit(leftContext),
        new Visitor().visit(rightContext),
        Optional.ofNullable(BINARY_OPERATORS.get(operatorName))
            .orElseGet(() -> BinaryOperatorType.fromSymbol(operatorName).getInstance()));

[johngrimes]

Consider this in the implementation of #2421, #2399, #2383, #2394, #2398.