rewrite web3-eth-iban

[frankiebee]

rewrite package to use modern es6
and inherit from base class

related #3923

[loredanacirstea]

@frankiebee, for external contributors, do you have a clear list of what changes the refactoring must target? I see #3904 - is there an even better refactoring example?

I can help with refactoring web3-eth-iban since no one else is assigned.

[frankiebee]

@loredanacirstea thanks for your PR #3955! we are still working on a good example of the rewrite, and #3904 is not quite hitting those requirements. We have big plans to improve the code and any help would be greatly appreciated. A really rough outline is in issue #3848 and i'll start collecting more of the notes that we have been discussing there over the next couple of days. From a glance your PR looks excellent and if it works well would love to merge it in but it won't satisfy closing this issue in its entirety as it stands now. But love the work! and i'll continue this conversation on the PR.

[loredanacirstea]

@frankiebee I understand from #3848 (comment) that I should base my rewrite PR on rewrite-core.
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