Add support for duplicate fieldnames in dom.Value#Struct

[desaikd]

Description of changes:
This PR is to support duplicate fields in a Struct according to the Ion Spec.

Tasks:

1. Added changes in the dom.Value#Struct for supporting duplicate fields. (All the Struct field will now be stored in (fieldname, values) -> [string, Value[]] form)
2. dom.Value#Struct constructor would accept both Iterable<string, Value> as well as Iterable<string, Value[]> for creating Struct.
3. Added getAll and allFields methods for getting all values of Struct fields.
4. toString method returns with all the values of Struct fields.
5. Changes in test suite for duplicate fields.
6. add tests for duplicate fields.

[desaikd]

Completed Tasks:

1. added changes for getAll() and allFields() methods.
2. changed ionEquals() method for checking unordered duplicate fields.
3. Tests suite changes for all the above tasks.

Questions:
Could you explain with an example on how the fields return value would look like? Also for the iterator?
e.g.

{foo: 1, baz: true, foo: 2} 

For above Struct value what should be the return value for fields() as well as iterator ?
Currently it will return something like this:

[['foo',[1,2]] , ['baz' , true]]

[desaikd]

Changes:

{foo: 1, baz: true, foo: 2} 

fields() would return Struct fields as below with only the last value:

[['foo', 2] , ['baz' , true]]

• To get all the field values allFields() can be used.
• toString method returns with all the values of Struct fields.