Use Niebler's suggestion for customization points for experimental::all()

[jaredhoberock]

nvcc may be having issues processing the way all() is called via ADL (see #304). The new-style customization point shouldn't have this issue.

[jaredhoberock]

all() should also provide a default implementation for Ranges that are already Views (the default would simply be the identity function). That requires some way to recognize when a Range is a View and not some other thing (like a Container).

Perhaps the default implementation should be to return some sort of range_view<Iterator,Sentinel> type? The special cases could be reserved for things which are known to have optimizations -- for example, Ranges with contiguous iterators.