This change adds support for creating nodes in a flow graph that support
multiple inputs and outputs directly, instead of our current approach
where we need to re-implement the `FlowConsumerContext` interface in
order to support multiple inputs or outputs.

This change updates the repository to remove the use of wildcard imports
everywhere. Wildcard imports are not allowed by default by Ktlint as
well as Google's Java style guide.

This change updates the project structure to become flattened.
Previously, the simulator, frontend and API each lived into their own
directory.

With this change, all modules of the project live in the top-level
directory of the repository. This should improve discoverability of
modules of the project.

This change makes the frontend independent of the API by removing the
static file serving logic from the API server. Instead, we can serve the
frontend as static HTML over CDNs.