Ensure apps with alternative bin names are handled properly

[thedodd]

Per the discussion over here #46 (comment), it would seem that Trunk needs to do a better job at determining the name of the output WASM. Let's leverage cargo-metadata as much as possible to handle edge cases like this.

This should be a simple matter of checking the package's targets (already provided by cargo-metadata), and then checking to ensure that there is only 1 bin. In the future, as we move to complete #46, we will support multiple bins, where one is the app, and others are workers.

[thedodd]

This is pretty closely related to #58, let's knock these both out together.

[thedodd]

Ultimately, we will need to take a more robust approach with this. We will need to implement the pattern described in #46 (comment) in order to fully resolve this issue.

#58 will implement a quick fix for other naming issues. #46 will add the more robust controls and will probably ship in 0.6.0 or so.