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 migrates the remainder of the codebase to the
SimulationCoroutineDispatcher implementation.

This change introduces the SimulationCoroutineDispatcher implementation
which replaces the TestCoroutineDispatcher for running single-threaded
simulations.

Previously, we used the TestCoroutineDispatcher from the
kotlinx-coroutines-test modules for running simulations. However, this
module is aimed at coroutine tests and not at simulations.

In particular, having to construct a Clock object each time for the
TestCoroutineDispatcher caused a lot of unnecessary lines. With the new
approach, the SimulationCoroutineDispatcher automatically exposes a
usable Clock object.

In addition to ergonomic benefits, the SimulationCoroutineDispatcher is
much faster than the TestCoroutineDispatcher due to the assumption that
simulations run in only a single thread. As a result, the dispatcher
does not need to perform synchronization and can use the fast
PriorityQueue implementation.

This change adds metrics that are tracked per function instance, which
includes the runtime of the invocations, the number of invocations
(total, warm, cold, failed).

This change exposes several metrics from the Serverless service, which
are needed for the experiments.

This change adds the initial implementation of OpenDC Serverless to the
main repository, which is based on Soufiane Jounaid's work. This commit
introduces the main interfaces into the repository. Later commits will
introduce the other features that Soufiane has implemented previously in
OpenDC.