Use value `0` for consistency with `MPI_DATATYPE_NULL` and increment
the index of other "op" constants.

Create a dedicated header to hold functions to convert between the Comm
encapsulation and MPI_Comm.  In the future the main Comm header will not
need to include raw MPI interfaces at all.

Move the MPI-backed implementation into a separate file.  Use an
abstraction layer with runtime dispatch so that the main Comm does not
need MPI.  Note that we still use a few MPI types and constants in the
Comm interface, but those can be refactored later.

It was used only inside our own implementation in one place, and its
implementation was just a thin wrapper around our `Allgather` interface,
so simply use that directly.

With our general `Gatherv` interface we no longer need a per-type
specialization of `GathervArrays`.

With our general `Gather` interface we no longer need a per-type
specialization of `GatherArrays`.

With our general `Reduce` interface we no longer need a per-type
specialization of `ReduceValues`.

With our general `Bcast` interface we no longer need a per-type
specialization of `BroadcastValue`.

With our general `Bcast` interface we no longer need a per-type
specialization of `BroadcastVector`.

Avoid explicitly passing `sizeof(T)` to `Bcast`.

Create a Comm implementation for serial programs.  Use implementations
inspired by `source/adios2/helper/mpidummy.cpp`.

If `CommFromMPI()` is given `MPI_COMM_NULL`, fall back to `CommDummy()`
instead.