Restructure and refactor the Topiary binary's source files

[Xophmeister]

This PR is preparatory, in aid of #11, to make a clear distinction between library and binary-related code. In particular, it allows for binary-related modules.

• Moved src/main.rs to src/bin/topiary/main.rs
• Factored out the OutputFile and SupportedLanguages types into their own modules.

Notes:

• This separation may also be useful for Make sure Topiary works when added as a crate in a third-party app #137 and topiary.tweag.io Interactive Playground #171.
• Cargo workspaces would be the next logical step -- so library and binary dependencies can be separated -- but I couldn't get it to work without breaking the Nix-related code. (I'll add this as an issue.)