Verification of contract libraries

[jmank88]

If your contracts use libraries, you will notice that the bytecode contains substrings of the form __$53aea86b7d70b31448b230b20ae141a537$__. These are placeholders for the actual library addresses. The placeholder is a 34 character prefix of the hex encoding of the keccak256 hash of the fully qualified library name. The bytecode file will also contain lines of the form // -> at the end to help identify which libraries the placeholders represent.

From https://solidity.readthedocs.io/en/v0.5.8/using-the-compiler.html

When verifying a contract, explorer should notice placeholder strings for libraries and relax the match from exact to legal-address. We should extract those addresses to present and link in the UI as well.

An alternative approach would be to allow the user to provide the name/address association, and to just compile with that. This would be simpler to implement, but much less automatic.