feat: adapt to latest did core spec and libs

[mirceanis]

This is more or less a full rewrite of the library, trying as much as possible to keep true to the previous design and API but with a few extras and also some necessary breaking changes.

• all transactions can now accept options so that it's easier to control gas cost.
• added ability to use private networks (closes #50)
• added ability to specify a transaction signer (so that it's no longer solely dependent on the provider)
• updated to the latest libraries and switched to ethers.js

BREAKING CHANGE: new did core spec uses different format for DID documents and resolution. BREAKING CHANGE: Now using ethers.js which means that providers and signers may have a different API BREAKING CHANGE: default export was removed

closes #50 closes #59 closes #21 closes #49 closes #51 closes #52 closes #57 closes #58

[uport-automation-bot]

:tada: This PR is included in version 2.0.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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