Add function for compiling base-2 log at compile time

ilog2.hh

+#ifndef OSV_ILOG2_HH
+#define OSV_ILOG2_HH
+
+#include <cstdint>
+
+constexpr unsigned ilog2_roundup_constexpr(std::uintmax_t n)
+{
+    return n <= 1 ? 0 : 1 + ilog2_roundup_constexpr((n >> 1) + (n & 1));
+}
+
+#endif