Add value_type to span (#425)

* Add value_type to span

Currently I'm working on project which involves a lot of `span`s and mocking via Google Mock. Unfortunately a lot of standard matchers requires `value_type` type definition inside container which `gsl::span` lacks.

This pull request add `value_type` type definition inside `gsl::span`

* Strip cv from value_type of span and span_iterator

1 files changed, 4 insertions, 1 deletions

diff --git a/include/gsl/span b/include/gsl/span
index 957c9e6..3a8d31d 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -133,7 +133,7 @@ namespace details
         using element_type_ = typename Span::element_type;
     public:
         using iterator_category = std::random_access_iterator_tag;
-        using value_type = std::remove_const_t<element_type_>;
+        using value_type = std::remove_cv_t<element_type_>;
         using difference_type = typename Span::index_type;
 
         using reference =
@@ -337,6 +337,7 @@ class span
 public:
     // constants and types
     using element_type = ElementType;
+    using value_type = std::remove_cv_t<ElementType>;    
     using index_type = std::ptrdiff_t;
     using pointer = element_type*;
     using reference = element_type&;
@@ -346,6 +347,8 @@ public:
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
 
+    using size_type = index_type;
+
     constexpr static const index_type extent = Extent;
 
     // [span.cons], span constructors, copy, assignment, and destructor