[plt-scheme] 203.10

The exp-tagged code in CVS for MzScheme and MrEd is now version 203.10.

The changes are minor, those the first one may cause headaches:

 * Changed column counting (in syntax objects and error messages) to
   start from 0. This seems to be more standard than counting from 1.

   Lines and positions still start with 1 --- which, again, seems to be
   the most standard choice, though it seems odd to me that lines and
   positions are counted differently than columns.

 * Added `vector->immutable-vector'.

 * Added `immutable-box'.

Matthew