Make Span compatible with std::Container

Needed this to play nice with one of the triangulation libraries I'm experimenting with (so I don't have to make vectors and/or copy data around).

Diffs=
d56be39f4 Make Span compatible with std::Container

.rive_head

diff --git a/.rive_head b/.rive_head
index 01d41b0..a229925 100644
--- a/.rive_head
+++ b/.rive_head
@@ -1 +1 @@
-758647ef64eb80195b5c7678671b254545491b42
+d56be39f44ce9d21121b088bcfc09b19ef3a1832

include/rive/span.hpp

diff --git a/include/rive/span.hpp b/include/rive/span.hpp
index c7e867b..ac899f4 100644
--- a/include/rive/span.hpp
+++ b/include/rive/span.hpp
@@ -58,6 +58,16 @@
             assert(size <= m_Size - offset);
             return {m_Ptr + offset, size};
         }
+
+        // Makes rive::Span std::Container compatible
+        // https://en.cppreference.com/w/cpp/named_req/Container
+        typedef typename std::remove_cv<T>::type value_type;
+        typedef T& reference;
+        typedef T const& const_reference;
+        typedef T* iterator;
+        typedef T const* const_iterator;
+        typedef std::ptrdiff_t difference_type;
+        typedef size_t size_type;
     };
 
     template <typename Container>