(for instance *p was not allowed on the left side of "="). Changed
to Expression everywhere (this is too liberal, UnaryExpr is probably
good enough, but it seems funny, and we need to check semantically
anyway). This matches 6g yacc.
- Write expression syntac recursively to express evaluation order
and precedence syntactically.
- Organized open issues list, folded in stuff from todo.txt which
is now obsolete.

R=r
DELTA=108  (41 added, 52 deleted, 15 changed)
OCL=16903
CL=16910