Update to proposal p0122r7

[martinmoene]

p0122 - span: bounds-safe views for sequences of objects

See also Reddit: 2018 San Diego ISO C++ Committee Trip Report.

See also 2019-02 Kona ISO C++ Committee Trip Report.

Plan:

• [x] Make index_type unsigned, see issue #21;
• [x] Remove (configure to enable) comparison operators; p1085: Should Span be Regular?
• [x] Add span::ssize() method to get the size of a container as a signed value; p1227: Signed size() functions
• [x] Add std::ssize() free function to get the size of a container as a signed value; p1227: Signed size() functions
• [x] Add std::size() free function to get the size of a container as an usigned value; n4017: Non-member size() and more

---

size(spn), ssize(spn)

In namespace nonstd::span_lite:

namespace nonstd { namespace span_lite {

template< class T, extent_t Extent /*= dynamic_extent*/ >
span_constexpr std::size_t size( span<T,Extent> const & spn )
{
    return static_cast<std::size_t>( spn.size() );
}

template< class T, extent_t Extent /*= dynamic_extent*/ >
span_constexpr std::ptrdiff_t ssize( span<T,Extent> const & spn )
{
    return static_cast<std::ptrdiff_t>( spn.size() );
}
}} // namespace

namespace nonstd { 
    using span_lite::size;
    using span_lite::ssize;
}