Add demo simulating a mail spam detection service

[dartdart26]

This demo simulates a service that checks if a mail is spam without the server seeing the mail or the result. If a mail contains "evil.url", it will be classified as spam.

Add test benches, an unit test and update the README.

Reference: https://vitalik.ca/general/2020/07/20/homomorphic.html

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[dartdart26]

@googlebot I signed it!