the interpreter and update code to use ast.BasicDecl and
multi-type switch.  There are still a lot of "switch _ :=
x.(type)" that should make use of the new type switch syntax,
but those will be a different CL.

R=rsc
APPROVED=rsc
DELTA=58  (16 added, 23 deleted, 19 changed)
OCL=34853
CL=34963