- Replaced large numbers in libraries by library-own constants (starting with "MAX_").
- Use numerical instead of string keys for positions in lib.soko.area.
- assert_type: New type descriptor "valid object" (with refman entry), and
use "valid object" instead of "object" in some appropriate functions.
- Extending use of assertions to api2init functions (i.e. arguments are now checked).
- Adding some forgotten assertions from r1602.
- Reintegrate lib.map._paste into lib.map.paste (missed in r1602).
- Typos and quotation marks in refman.
