Change unique_span to take a deleter object taking a span, not a template param accepting taking pointer

[eyalroz]

unique_span deletion - destruction and de-allocation - is more complex than one might think. With trivially-constructible types, we can supposed just use new T[size] to start off and then delete[] eventually. But in fact, the code which needs to run when we perform raw allocation and placement-new construction - a-la std::vector - is more complex, and definitely depends on array size. It is problematic to presume we know how to delete given only a pointer and no size, and unable to make a last-minute call about this. Specifically, the current setup makes it impossible to determine how many placement-destruct's are necessary given a pointer.

Moreover, the bespoke deleter causes unique_span's to be nearly non-interchangeable, because their types are deleter-dependent.

Even though it may cost us a few more bytes, let's:

• Remove the extra template parameter from `unique_span<T, Deleter> (this means the memory::XXXX:unique_span's will now be just unique_span's)
• Use a 'universal' deleter type - which can't just be instantiated
• Offer a default, simple, actual deleter
• Offer span uspan makers/generators of fixed value and by function.