🐛 Fix fallback implementation of bit_ceil

Problem:
- The fallback implementation of `bit_ceil` is implemented incorrectly. In
  particular it returns the wrong values for exact powers of 2.

Solution:
- Fix the calculation: subtracting 1 before calculating the bit width is
  appropriate.

Notes:
- Thanks to Walter Brown for pointing out this issue in private correspondence.
- Undoubtedly, this was missed because of issue #117.

Author: elbeno

include/stdx/bit.hpp

@@ -223,7 +223,7 @@ template <typename T>
     if (x <= 1U) {
         return 1U;
     }
-    return T(1U << bit_width(x));
+    return T(1U << bit_width(x - 1U));
 }
 
 template <typename T>

test/bit.cpp

@@ -160,13 +160,19 @@ TEMPLATE_TEST_CASE("bit_ceil", "[bit]", std::uint8_t, std::uint16_t,
                    std::uint32_t, std::uint64_t) {
     STATIC_REQUIRE(stdx::bit_ceil(TestType{}) == 1);
     STATIC_REQUIRE(stdx::bit_ceil(TestType{1u}) == 1);
+    STATIC_REQUIRE(stdx::bit_ceil(TestType{3u}) == 4);
+    STATIC_REQUIRE(stdx::bit_ceil(TestType{4u}) == 4);
+    STATIC_REQUIRE(stdx::bit_ceil(TestType{5u}) == 8);
     STATIC_REQUIRE(stdx::bit_ceil(TestType{45u}) == 64);
 }
 
 TEMPLATE_TEST_CASE("bit_floor", "[bit]", std::uint8_t, std::uint16_t,
                    std::uint32_t, std::uint64_t) {
     STATIC_REQUIRE(stdx::bit_floor(TestType{}) == 0);
     STATIC_REQUIRE(stdx::bit_floor(TestType{1u}) == 1);
+    STATIC_REQUIRE(stdx::bit_floor(TestType{3u}) == 2);
+    STATIC_REQUIRE(stdx::bit_floor(TestType{4u}) == 4);
+    STATIC_REQUIRE(stdx::bit_floor(TestType{5u}) == 4);
     STATIC_REQUIRE(stdx::bit_floor(TestType{45u}) == 32);
 }