Reworked span to match GSL design.

include/gsl_assert.h

@@ -55,9 +55,9 @@ struct fail_fast : public std::runtime_error
 }
 
 #define Expects(cond)  if (!(cond)) \
-    throw gsl::fail_fast("GSL: Precondition failure at " __FILE__ GSL_STRINGIFY(__LINE__));
+    throw gsl::fail_fast("GSL: Precondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
 #define Ensures(cond)  if (!(cond)) \
-    throw gsl::fail_fast("GSL: Postcondition failure at " __FILE__ GSL_STRINGIFY(__LINE__));
+    throw gsl::fail_fast("GSL: Postcondition failure at " __FILE__ ": " GSL_STRINGIFY(__LINE__));
 
 
 #elif defined(GSL_TERMINATE_ON_CONTRACT_VIOLATION)

include/gsl_util.h

@@ -76,23 +76,23 @@ private:
 
 // finally() - convenience function to generate a final_act
 template <class F>
-final_act<F> finally(const F &f)
+inline final_act<F> finally(const F &f)
 noexcept { return final_act<F>(f); }
 
 template <class F>
-final_act<F> finally(F &&f) noexcept
+inline final_act<F> finally(F &&f) noexcept
 { return final_act<F>(std::forward<F>(f)); }
 
 // narrow_cast(): a searchable way to do narrowing casts of values
 template<class T, class U>
-constexpr T narrow_cast(U u) noexcept
+inline constexpr T narrow_cast(U u) noexcept
 { return static_cast<T>(u); }
 
 struct narrowing_error : public std::exception {};
 
 // narrow() : a checked version of narrow_cast() that throws if the cast changed the value
 template<class T, class U>
-T narrow(U u)
+inline T narrow(U u)
 { T t = narrow_cast<T>(u); if (static_cast<U>(t) != u) throw narrowing_error(); return t; }
 
 //