ENH: extend mpiAllGather to include integer and float types

- was previously limited to 'char' whereas gatherv/scatterv
  already supported various integer and float types

STYLE: rebundle allToAll declarations with macros

ENH: provide a version of allToAllConsensus returning the Map

- simplifies use and avoids ambiguities in the send/recv parameters

- the Map version will now also transmit zero value data if they exist
  in the Map. Unlike the List version, zero values are not necessary to
  signal connectivity with a Map.

COMP: forwarding template parameters for NBX routines

ENH: consolidate PstreamBuffers size exchange options

- had a variety of nearly identical backends for all-to-all,
  gather/scatter. Now combined internally with a dispatch enumeration
  which provides better control over which size exchange algorithm
  is used.

DEFEATURE: remove experimental full-NBX PstreamBuffers variant

- no advantages seen compared to the hybrid NBX/PEX approach.
  Removal reduces some code cruft.

DEFEATURE: remove experimental "double non-blocking" NBX version

- the idea was to avoid blocking receives for very large data transfers,
  but that is usually better accomplished with a hybrid NBX/PEX approach
  like PstreamBuffers allows