make Pos smaller

reduces peak hep memory use on eval of our test system from 264.4MB to 242.3MB,
possibly also a slight performance boost.

theoretically memory use could be cut down by another eight bytes per Pos on
average by turning it into a tuple containing an index into a global base
position table with row and column offsets, but that doesn't seem worth the
effort at this point.

src/libutil/error.hh

@@ -53,6 +53,7 @@ typedef enum {
     lvlVomit
 } Verbosity;
 
+/* adjust Pos::origin bit width when adding stuff here */
 typedef enum {
     foFile,
     foStdin,