[polyml] Changes to Foreign structure

There have been a few changes to the Foreign structure. The callN functions have been renamed as buildCallN and the way functions are passed as arguments has been changed.

The reason for the changes is to make clear that the expensive operations are creating the C functions and closures and that calling a C function or passing a constructed closure are comparatively cheap.

It is important to call the buildXXX functions at the top-level e.g. in a structure, so that the C function is created once. The old callN functions were curried and it wasn't apparent that the partial application to the conversions was quite different to the application of this to the arguments. For that reason the buildXXX take a tuple.

David