lib: move `examples/mpi` to `lib/mpi/examples`

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/examples/mpi/Makefile b/lib/mpi/examples/Makefile
similarity index 100%
rename from examples/mpi/Makefile
rename to lib/mpi/examples/Makefile

diff --git a/examples/mpi/README.md b/lib/mpi/examples/README.md
similarity index 100%
rename from examples/mpi/README.md
rename to lib/mpi/examples/README.md

diff --git a/examples/mpi/hosts.example b/lib/mpi/examples/hosts.example
similarity index 100%
rename from examples/mpi/hosts.example
rename to lib/mpi/examples/hosts.example

diff --git a/examples/mpi/src/mpi_simple.nit b/lib/mpi/examples/src/mpi_simple.nit
similarity index 100%
rename from examples/mpi/src/mpi_simple.nit
rename to lib/mpi/examples/src/mpi_simple.nit