Add different request types (#677)

Author: simonbyrne

docs/src/reference/pointtopoint.md

@@ -3,7 +3,11 @@
 ## Types
 
 ```@docs
+MPI.AbstractRequest
 MPI.Request
+MPI.UnsafeRequest
+MPI.MultiRequest
+MPI.UnsafeMultiRequest
 MPI.RequestSet
 MPI.Status
 ```
@@ -59,6 +63,7 @@ MPI.Waitsome
 ### Probe/Cancel
 
 ```@docs
+MPI.isnull
 MPI.Cancel!
 MPI.Iprobe
 MPI.Probe

src/collective.jl

@@ -17,7 +17,7 @@ function Barrier(comm::Comm)
 end
 
 """
-    Ibarrier(comm::Comm)
+    Ibarrier(comm::Comm[, req::AbstractRequest = Request())
 
 Blocks until `comm` is synchronized.
 
@@ -28,8 +28,8 @@ If `comm` is an intercommunicator, then it blocks until all members of the other
 # External links
 $(_doc_external("MPI_Ibarrier"))
 """
-function Ibarrier(comm::Comm)
-    req = Request()
+function Ibarrier(comm::Comm, req::AbstractRequest = Request())
+    @assert isnull(req)
     # int MPI_Ibarrier(MPI_Comm comm, MPI_Req req)
     API.MPI_Ibarrier(comm, req)
     return req