relay mpi 'any-to-any'

While thinking about a mpi all-to-all interface, @bryujin and I talked over several potential interfaces.

The best candidate surviving is one that constructs node trees that include destination ranks.

For example, given an MPI job w/ three ranks, where we want to perform the following exchange:

```
Rank 0 sends A to Rank 1 and Rank 2
Rank 0 sends B to Rank 2
Rank 1 sends C to Rank 2
Rank 2 sends D to Rank 0

```
Callers on the ranks would construct input nodes that look like the following:

```
Rank 0:
[
{ "dest" : [1,2] , "value" : (A)},
{ "dest" : [2] , "value" : (B)}
]

Rank 1:
[
{ "dest" : [2] , "value" : (C)}
]

Rank 2:
[
{ "dest" : [0] , "value" : (D)}
]
```
set_external() can used to zero-copy the data (with some cost to copy the meta-data)

And the output would look like the following:

```
Rank 0:
[ (D) ]

Rank 1:
[ (A) ]

Rank 2:
[ (A,B,C) ]

```
This isn't really a generic all to all interface. (But I belive it can be implemented with an all-to-all).
We will have to think about the proper name for this. 'any-to-any' is a place-holder, which may be good enough.

Start by reviewing the proposed input and output node shapes and the set_external() zero-copy behavior described in the issue. Resolve the interface name and semantics for the any-to-any exchange before implementation; done should include an agreed API that supports the rank-exchange example.

Written by the indexing model from the issue text.