Cleanup additional GCC warnings

When turning on the following flags, several additional warnings
were generated, which have been cleaned up in this patch. The
flags included:

-Wextra
-Wpedantic
-Wconversion
-Wsign-conversion
-Wctor-dtor-privacy
-Wshadow
-Wnon-virtual-dtor
-Wold-style-cast
-Wcast-align
-Woverloaded-virtual

gsl/multi_span

@@ -1451,7 +1451,7 @@ public:
     constexpr size_type length() const noexcept { return this->size(); }
 
     // length of the multi_span in bytes
-    constexpr size_type size_bytes() const noexcept { return sizeof(value_type) * this->size(); }
+    constexpr size_type size_bytes() const noexcept { return narrow_cast<size_type>(sizeof(value_type)) * this->size(); }
 
     // length of the multi_span in bytes
     constexpr size_type length_bytes() const noexcept { return this->size_bytes(); }
@@ -1653,7 +1653,8 @@ constexpr auto as_multi_span(multi_span<const byte, Dimensions...> s) noexcept -
              ConstByteSpan::bounds_type::static_size % narrow_cast<std::ptrdiff_t>(sizeof(U)) == 0),
         "Target type must be a trivial type and its size must match the byte array size");
 
-    Expects((s.size_bytes() % sizeof(U)) == 0 && (s.size_bytes() / sizeof(U)) < PTRDIFF_MAX);
+    Expects((s.size_bytes() % narrow_cast<std::ptrdiff_t>(sizeof(U))) == 0 && 
+            (s.size_bytes() / narrow_cast<std::ptrdiff_t>(sizeof(U))) < PTRDIFF_MAX);
     return {reinterpret_cast<const U*>(s.data()),
             s.size_bytes() / narrow_cast<std::ptrdiff_t>(sizeof(U))};
 }

gsl/span

@@ -494,7 +494,7 @@ public:
     constexpr index_type length() const noexcept { return size(); }
     constexpr index_type size() const noexcept { return storage_.size(); }
     constexpr index_type length_bytes() const noexcept { return size_bytes(); }
-    constexpr index_type size_bytes() const noexcept { return size() * sizeof(element_type); }
+    constexpr index_type size_bytes() const noexcept { return size() * narrow_cast<index_type>(sizeof(element_type)); }
     constexpr bool empty() const noexcept { return size() == 0; }
 
     // [span.elem], span element access

gsl/string_span

@@ -331,7 +331,7 @@ public:
     // Container signature should work for basic_string after C++17 version exists
     template <class Traits, class Allocator>
     constexpr basic_string_span(std::basic_string<element_type, Traits, Allocator>& str)
-        : span_(&str[0], str.length())
+        : span_(&str[0], narrow_cast<std::ptrdiff_t>(str.length()))
     {
     }