By Tobias Carroll There’s a bit of investment advice that’s often attributed to Mark Twain, and it’s relatively straightforward: “Buy land, they’re not making it anymore.” Unless he had access to a time machine, Twain was probably not thinking of Singapore in the year 2022 when he made his comments. Nonetheless, the country’s real estate market makes for an almost perfect illustration of the adage in question. Singapore is, you’ll remember, one of the most densely populated countries in the world. A 2011 BBC report noted that it’s second only to Monaco in terms of crowded city-states — and whe…