[ethers] Factory for connecting to existing contract instances

[quezak]

This is a follow-up for PR #160 (CC issue #137)

Problem: existing methods allowed only to deploy a new contract instance, or attach to an existing address, but only connecting through a complete Signer / Wallet. In actual use cases, you often don't need (or even don't want) the contract instance to have a signer connected, just a provider, to use contract's view or pure functions.

Proposed solution:

• add to the contract factories: static connect(address: string, signerOrProvider: Signer | Provider) -- this just invokes the Contract constructor with the same params and the hardcoded ABI (since we can't add runtime functions to our contract type declarations)
• for abstract contracts (or if bytecode was just not provided), generate a simplified version of the factory, containing just the above static method, without the usual deployment methods.

[coveralls]

Pull Request Test Coverage Report for Build 232

• 15 of 17 (88.24%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 90.336%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/targets/ethers/index.ts	6	8	75.0%

Totals
Change from base Build 229:	0.9%
Covered Lines:	454
Relevant Lines:	488

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💛 - Coveralls

[krzkaczor]

Published as typechain@0.3.19