Add nothrow @nogc.

[SFrijters]

No description provided.

[John-Colvin]

so we are insisting that any callbacks passed in are nothrow and nogc?

[John-Colvin]

A first step should be this:
Everything should be @nogc nothrow except for the function type aliases (e.g. MPI_Copy_function) and any functions that take parameters of these types (e.g. MPI_Comm_create_errhandler).

Dealing with making those conditionally @nogc nothrow depending on argument types etc is something that can be dealt with later