We'd like to change the data layout of `interval` to save space.
As a result, start() and end() which return references to data
members must return objects (not references). Since we'd like
to maintain zero-copy for these functions, we change them to
return objects containing references (rather than references
to objects), avoiding copying of potentially expensive objects.
We repurpose the interval_bound class to hold references (by
instantiating it with `const T&` instead of `T`) and provide
converting constructors. To make transform_bounds() retain
zero-copy, we add start() and end() that take *this by
rvalue reference.
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