This change renames the opendc-serverless module to opendc-faas to
better distinguish between the two terms (Serverless and FaaS) and be
clearer about the intent of the module.

The opendc-faas module holds the code for the FaaS platform on top of
OpenDC. Although this is one approach of doing serverless, serverless
can also entail other services that will not be covered by this module.

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.