Add `Equatable` conformance to `RESPValue`

Nathan Harrisrequested to merge
respvalue-equatable into master

Motivation:

While working on unit tests the need for conformance to Equatable for RESPValue has been needed a few times and it was decided to make it public.

Modifications:

Added conformance to Equatable for RESPValue with unit test.

Result:

Users should now be able to compare two RESPValue instances for equality.

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