CLEANUP: Export fewer functions

Currently, **future** exports quite a few functions beyond the _Future API_ itself. Some of them are used by backends but not really needed by uses of **future** per se - can they be removed from the public API?

For example, **future.callr** use `MultiprocessFuture`, which belongs more to a "protected" backend API. Maybe there's a robust way to make those available without exporting them, e.g.
```r
MultiprocessFuture <- future::import_protected("MultiprocessFuture")
```
OTH, with such an approach we'll lose all the advantages that come with `R CMD check`, e.g. code inspection. Maybe that's ok.

Begin by auditing the future package's public exports and the backend use of MultiprocessFuture, including future.callr. Done requires a decided scope for which exports belong to the Future API and a validated approach for backend access without losing R CMD check benefits.

Written by the indexing model from the issue text.