[wpiutil] Add std::span implementation

Imported from https://github.com/tcbrindle/span with ifdef's removed
(as we require C++17).

wpiutil/src/test/native/cpp/span/test_deduction_guides.cpp

@@ -0,0 +1,98 @@
+
+#include "wpi/span.h"
+
+#include "gtest/gtest.h"
+
+#include <vector>
+
+using wpi::span;
+
+namespace {
+
+template <typename R1, typename R2>
+constexpr bool equal(R1&& r1, R2&& r2)
+{
+    auto first1 = std::begin(r1);
+    const auto last1 = std::end(r1);
+    auto first2 = std::begin(r2);
+    const auto last2 = std::end(r2);
+
+    while (first1 != last1 && first2 != last2) {
+        if (*first1 != *first2) {
+            return false;
+        }
+        ++first1;
+        ++first2;
+    }
+
+    return true;
+}
+
+constexpr bool test_raw_array()
+{
+    int arr[] = {1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<int, 3>>);
+
+    return equal(arr, s);
+}
+
+constexpr bool test_const_raw_array()
+{
+    constexpr int arr[] = {1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<const int, 3>>);
+
+    return equal(arr, s);
+}
+
+constexpr bool test_std_array()
+{
+    auto arr = std::array<int, 3>{1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<int, 3>>);
+
+    return equal(arr, s);
+}
+
+constexpr bool test_const_std_array()
+{
+    const auto arr = std::array<int, 3>{1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<const int, 3>>);
+
+    return equal(arr, s);
+}
+
+bool test_vec()
+{
+    auto arr = std::vector{1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<int>>);
+
+    return equal(arr, s);
+}
+
+bool test_const_vec()
+{
+    const auto arr = std::vector{1, 2, 3};
+    auto s = span{arr};
+    static_assert(std::is_same_v<decltype(s), span<const int>>);
+
+    return equal(arr, s);
+}
+
+}
+
+TEST(Deduction, FromRawArrays)
+{
+    static_assert(test_raw_array());
+    static_assert(test_const_raw_array());
+    static_assert(test_std_array());
+    static_assert(test_const_std_array());
+
+    ASSERT_TRUE(test_std_array());
+    ASSERT_TRUE(test_const_std_array());
+    ASSERT_TRUE(test_vec());
+    ASSERT_TRUE(test_const_vec());
+}