Fix declared and not used errors and unused import errors in

Fix declared and not used errors and unused import errors in
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