Solidity by example demos

Implement the following Solidity example contracts in Fe:

- [Voting](https://docs.soliditylang.org/en/latest/solidity-by-example.html#voting)
- [Simple Open Auction](https://docs.soliditylang.org/en/latest/solidity-by-example.html#simple-open-auction)
- [Blind Auction](https://docs.soliditylang.org/en/latest/solidity-by-example.html#id2)
- [Safe Remote Purchase](https://docs.soliditylang.org/en/latest/solidity-by-example.html#safe-remote-purchase)

Use the four linked Solidity by Example pages as the specification: Voting, Simple Open Auction, Blind Auction, and Safe Remote Purchase. Then locate the corresponding Fe example location and any existing validation entry point; the issue names no files or tests. Done means all four contracts are implemented in Fe and their behavior is verified against the linked examples.

Written by the indexing model from the issue text.