Make use of bytecode metadata to improve confirmation dialog

The Solidity compiler stores the hash of contract metadata in the bytecode of each deployed contract. This metadata could be used to improve the confirmation dialog to display the natspec description, decode the function call arguments and display a link to the source code.

Details: https://chriseth.github.io/notes/talks/trust_gap_meetup


cc @frankiebee @rekmarks

No files, tests, or entry points are named. Start by reading the confirmation-dialog behavior and the linked Details talk, then map how deployed-contract bytecode metadata could provide NatSpec text, decoded arguments, and a source-code link. Done means the dialog uses that metadata for all three improvements.

Written by the indexing model from the issue text.