Move printSize() into libutil

Also always include the unit (i.e. "MiB" instead of "M").

src/libutil/util.hh

@@ -137,6 +137,12 @@ N string2IntWithUnitPrefix(std::string_view s)
     throw UsageError("'%s' is not an integer", s);
 }
 
+/**
+ * Pretty-print a byte value, e.g. 12433615056 is rendered as `11.6
+ * GiB`.
+ */
+std::string renderSize(uint64_t value);
+
 /**
  * Parse a string into a float.
  */