Add an allElements() method alongside elements()

[zslayton]

v4.2.0 added support for structs with repeated field names, e.g.

{
  foo: 1,
  foo: true,
  foo: bar
}

To preserve backwards compatibility, existing struct accessor methods (get, fields) continued to return deduplicated results, using the value associated with the last appearance of each field name. New flavors of those methods (getAll, allFields) were added that would instead return an array with every value associated each field name.

We have not yet added an allElements method for the elements method. A call to elements with the sample struct above would return [bar] while the allElements method would return [1, true, bar].

[desaikd]

Express allElements() behavior in elements() with v5.0.0 release.

[desaikd]

fixed with #754