Boil down vmap! variants, extend to reduce & mapreduce

There's a whole menagerie of `vmap`-related functions exported, corresponding to mutating/non-mutating, temporal/non-temporal stores, and singlethreaded/multithreaded. Eight versions of the same function are maintainable, but if some other boolean switch is added, that's sixteen, which could get unwieldy. Wrapper types might allow for a more Julian interface - something like
```
map!(Threaded(+), Nontemporal(out), a, b)
```
to replace
```
vmapntt!(+, out, a, b)
```
The introduction of `Threaded(f)` could be extended to `reduce` & `mapreduce` for a simple multithreaded reduction interface.

Start by reading the existing exported vmap-related entry points, including vmapntt!, along with the reduce and mapreduce interfaces. Define how wrapper types such as Threaded and Nontemporal should replace the current variants, then verify that the consolidated map! API and threaded reductions preserve the existing behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.