Exercise: macros to constexpr

Replace the three macros in the file:

• kMaxUsers with a constexpr int constant equal to 100;
• square(x) with the function constexpr int square(int x), which returns the square of x;
• half(x) with the function constexpr int half(int x), which returns x / 2.

Keep the names: the tests use them and also check that kMaxUsers, square and half are no longer macros. The line #if defined(square) in the tests checks whether such a macro is defined.

• square(2 + 1) → 9
• 100 / square(5) → 4
• half(10 + 4) → 7

Constraints: x is any int value, negative ones too, and its square fits in an int; the tests compute one call of square at compile time.

Do not add extra #include lines unless the task says so explicitly.