Minor interface-breaking changes

• single_view returns by const & (see #817).
• reverse_view of a non-Sized, non-Bounded RandomAccess range (eg., a null-terminated string) no longer satisfies SizedRange.
• The generate and generate_n views now return the generated values by xvalue reference (T &&) to the value cached within the view (see #905).
• Views no longer prefer returning constant iterators when they can; some views have different constant and mutable iterators.

Enhancements

• Views can successfully adapt other views that have different constant and mutable iterators.
• The single, empty, and reverse views are much closer to the versions as specified in P0896.

Bug fixes

• "single_view should not copy the value" #817.
• "Calling back() on strided range does not return the correct last value in range" #901.
• "generate(foo) | take(n) calls foo n+1 times" #819.
• "generate seems broken with move-only return types" #905.
• "Unexpected behavior in generate with return by reference" #807.
• "Inconsistent behaviour of ranges::distance with ranges::view::zip using infinite views." #783.
• "Infinite loop when using ranges::view::cycle with an infinite range" #780.
• "Composing ranges::view::cycle with ranges::view::slice" #778.
• "cartesian_product view, now with moar bugs." #919.