Add "gsl" to #includes (#1184)

Office is seeing build breaks due to `#include "span"` including
C++20 span instead of gsl/span. Most likely we want all headers
includes qualified with "gsl/" to avoid similar issues.

include/gsl/algorithm

@@ -17,8 +17,8 @@
 #ifndef GSL_ALGORITHM_H
 #define GSL_ALGORITHM_H
 
-#include "assert" // for Expects
-#include "span"   // for dynamic_extent, span
+#include "gsl/assert" // for Expects
+#include "gsl/span"   // for dynamic_extent, span
 
 #include <algorithm>   // for copy_n
 #include <cstddef>     // for ptrdiff_t

include/gsl/gsl

@@ -18,16 +18,16 @@
 #define GSL_GSL_H
 
 // IWYU pragma: begin_exports
-#include "algorithm"   // copy
-#include "assert"      // Ensures/Expects
-#include "byte"        // byte
-#include "pointers"    // owner, not_null
-#include "span"        // span
-#include "zstring"     // zstring
-#include "util"        // finally()/narrow_cast()...
+#include "gsl/algorithm"   // copy
+#include "gsl/assert"      // Ensures/Expects
+#include "gsl/byte"        // byte
+#include "gsl/pointers"    // owner, not_null
+#include "gsl/span"        // span
+#include "gsl/zstring"     // zstring
+#include "gsl/util"        // finally()/narrow_cast()...
 
 #ifdef __cpp_exceptions
-#include "narrow" // narrow()
+#include "gsl/narrow" // narrow()
 #endif
 // IWYU pragma: end_exports
 

include/gsl/narrow

@@ -16,8 +16,8 @@
 
 #ifndef GSL_NARROW_H
 #define GSL_NARROW_H
-#include "assert"    // for GSL_SUPPRESS
-#include "util"      // for narrow_cast
+#include "gsl/assert"    // for GSL_SUPPRESS
+#include "gsl/util"      // for narrow_cast
 #include <exception> // for std::exception
 namespace gsl
 {

include/gsl/pointers

@@ -17,7 +17,7 @@
 #ifndef GSL_POINTERS_H
 #define GSL_POINTERS_H
 
-#include "assert" // for Ensures, Expects
+#include "gsl/assert" // for Ensures, Expects
 
 #include <cstddef>      // for ptrdiff_t, nullptr_t, size_t
 #include <functional>   // for less, greater

include/gsl/span

@@ -17,10 +17,10 @@
 #ifndef GSL_SPAN_H
 #define GSL_SPAN_H
 
-#include "assert"   // for Expects
-#include "byte"     // for byte
-#include "span_ext" // for span specialization of gsl::at and other span-related extensions
-#include "util"     // for narrow_cast
+#include "gsl/assert"   // for Expects
+#include "gsl/byte"     // for byte
+#include "gsl/span_ext" // for span specialization of gsl::at and other span-related extensions
+#include "gsl/util"     // for narrow_cast
 
 #include <array>       // for array
 #include <cstddef>     // for ptrdiff_t, size_t, nullptr_t

include/gsl/span_ext

@@ -27,8 +27,8 @@
 //
 ///////////////////////////////////////////////////////////////////////////////
 
-#include "assert" // GSL_KERNEL_MODE
-#include "util"   // for narrow_cast, narrow
+#include "gsl/assert" // GSL_KERNEL_MODE
+#include "gsl/util"   // for narrow_cast, narrow
 
 #include <cstddef> // for ptrdiff_t, size_t
 #include <utility>

include/gsl/string_span

@@ -1,4 +1,4 @@
 #pragma once
 #pragma message(                                                                                   \
     "This header will soon be removed. Use <gsl/zstring> instead of <gsl/string_span>")
-#include "zstring"
+#include "gsl/zstring"

include/gsl/util

@@ -17,7 +17,7 @@
 #ifndef GSL_UTIL_H
 #define GSL_UTIL_H
 
-#include "assert" // for Expects
+#include "gsl/assert" // for Expects
 
 #include <array>
 #include <cstddef>          // for ptrdiff_t, size_t

include/gsl/zstring

@@ -17,7 +17,7 @@
 #ifndef GSL_ZSTRING_H
 #define GSL_ZSTRING_H
 
-#include "span_ext" // for dynamic_extent
+#include "gsl/span_ext" // for dynamic_extent
 
 #include <cstddef>   // for size_t, nullptr_t