Rename simple_https_client/src/tls13client.rs to simple_https_client/src/main.rs?

[duesee]

Just testing out the discussions here... Can we rename simple_https_client/src/tls13client.rs to simple_https_client/src/main.rs? It's not a big deal, but I think it is good practice to stick to a structure newcomers can directly recognize, i.e., to use Convention over configuration.