Use <=> operator for base_uint, Issue, and Book: (#4411)

- Implement the `operator==` and the `operator<=>` (aka the spaceship
  operator) in `base_uint`, `Issue`, and `Book`. 
- C++20-compliant compilers automatically provide the remaining
  comparison operators (e.g. `operator<`, `operator<=`, ...).
- Remove the function compare() because it is no longer needed.
- Maintain the same semantics as the existing code.
- Add some unit tests to gain further confidence.
- Fix #2525.

src/ripple/protocol/impl/Book.cpp

@@ -47,58 +47,4 @@ reversed(Book const& book)
     return Book(book.out, book.in);
 }
 
-/** Ordered comparison. */
-int
-compare(Book const& lhs, Book const& rhs)
-{
-    int const diff(compare(lhs.in, rhs.in));
-    if (diff != 0)
-        return diff;
-    return compare(lhs.out, rhs.out);
-}
-
-/** Equality comparison. */
-/** @{ */
-bool
-operator==(Book const& lhs, Book const& rhs)
-{
-    return (lhs.in == rhs.in) && (lhs.out == rhs.out);
-}
-
-bool
-operator!=(Book const& lhs, Book const& rhs)
-{
-    return (lhs.in != rhs.in) || (lhs.out != rhs.out);
-}
-/** @} */
-
-/** Strict weak ordering. */
-/** @{ */
-bool
-operator<(Book const& lhs, Book const& rhs)
-{
-    int const diff(compare(lhs.in, rhs.in));
-    if (diff != 0)
-        return diff < 0;
-    return lhs.out < rhs.out;
-}
-
-bool
-operator>(Book const& lhs, Book const& rhs)
-{
-    return rhs < lhs;
-}
-
-bool
-operator>=(Book const& lhs, Book const& rhs)
-{
-    return !(lhs < rhs);
-}
-
-bool
-operator<=(Book const& lhs, Book const& rhs)
-{
-    return !(rhs < lhs);
-}
-
 }  // namespace ripple