cleanup in preparation for new scoping.

cleanup in preparation for new scoping.

walkstate -> walkstmt
walktype -> walkexpr; stmts moved to walkstmt

walktype and friends have a final Node **init
argument that can have side effects appended,
making it more explicit when they do and do not happen.
this replaces the old global addtop and addtotop.

delete switch map and interface conversion cases
(dropped from the language months ago).

R=ken
OCL=31465
CL=31468