Big count support for datatypes

[devreal]

Utilize the count and disp arrays for type-punning of int and MPI_Count arguments to datatype creation functions. Store and pack integer or size_t depending on what is needed. Adjust places where bigcount support for datatypes was missing.

This is work in progress and not yet properly tested, other than compiling through it. I wanted to put it out to get some eyes on it to see whether people agree with the design and that it's the right direction, before putting more effort into testing.