mstd::span rename index_type to size_type according to spec

Author: Marc Emmers

platform/cxxsupport/mstd_span

@@ -139,7 +139,7 @@ class span {
 public:
     using element_type = ElementType;
     using value_type = typename mstd::remove_cv_t<element_type>;
-    using index_type = size_t;
+    using size_type = size_t;
     using difference_type = ptrdiff_t;
     using pointer = element_type *;
     using const_pointer = const element_type *;
@@ -148,7 +148,7 @@ public:
     using iterator = pointer;
     using reverse_iterator = std::reverse_iterator<iterator>;
 
-    static constexpr index_type extent = Extent;
+    static constexpr size_type extent = Extent;
 
     // Constructors, copy and assignment
     template<size_t E = Extent,
@@ -160,7 +160,7 @@ public:
         typename mstd::enable_if_t<mstd::is_convertible<
         remove_reference_t<mstd::iter_reference_t<It>>(*)[],
         ElementType(*)[]>::value, int> = 0>
-    constexpr span(It ptr, index_type count) :
+    constexpr span(It ptr, size_type count) :
         _storage(ptr, count)
     {
         MBED_ASSERT(extent == dynamic_extent || extent == count);
@@ -251,32 +251,32 @@ public:
         return {data() + Offset, Count != dynamic_extent ? Count : size() - Offset};
     }
 
-    constexpr span<element_type, dynamic_extent> first(index_type count) const
+    constexpr span<element_type, dynamic_extent> first(size_type count) const
     {
         MBED_ASSERT(count <= size());
         return {data(), count};
     }
 
-    constexpr span<element_type, dynamic_extent> last(index_type count) const
+    constexpr span<element_type, dynamic_extent> last(size_type count) const
     {
         MBED_ASSERT(count <= size());
         return {data() + (size() - count), count};
     }
 
     constexpr span<element_type, dynamic_extent>
-        subspan(index_type offset, index_type count = dynamic_extent) const
+        subspan(size_type offset, size_type count = dynamic_extent) const
     {
         MBED_ASSERT(offset <= size() && (count == dynamic_extent || count <= size() - offset));
         return {data() + offset, count == dynamic_extent ? size() - offset : count };
     }
 
     // Observers
-    constexpr index_type size() const noexcept
+    constexpr size_type size() const noexcept
     {
         return _storage._size;
     }
 
-    constexpr index_type size_bytes() const noexcept
+    constexpr size_type size_bytes() const noexcept
     {
         return size() * sizeof(element_type);
     }
@@ -287,7 +287,7 @@ public:
     }
 
     // Element access
-    constexpr reference operator[](index_type idx) const
+    constexpr reference operator[](size_type idx) const
     {
         MBED_ASSERT(idx < size());
         return *(data() + idx);
@@ -336,7 +336,7 @@ private:
 };
 
 template<typename ElementType, size_t Extent>
-constexpr span<ElementType, Extent>::index_type span<ElementType, Extent>::extent;
+constexpr span<ElementType, Extent>::size_type span<ElementType, Extent>::extent;
 
 #if __cplusplus >= 201703L || __cpp_deduction_guides >= 201703L
 // Deduction guides