Low-level Rise sanity check

[Bastacyclop]

After #198 and #220, me and @johanneslenfers concluded that it would be very useful to implement some low-level Rise program sanity checks:

• for example, parallelism hierarchy should be respected, and mapGlobal(0)(mapGlobal(0)(...)) is not a valid low-level program
• it is possible to create writing races by using mapSeq/reduceSeq patterns in the wrong place (basically if multiple threads execute it), I think such programs should not be considered valid.

[Bastacyclop]

Example of writing race:

    val e = depFun((n: Nat) => fun(n`.`f32)(in =>
      in |> split(128) |> mapWorkGroup(
        mapLocal(id) >> toLocal >> mapSeq(id)
      ) |> join
    ))