topdown: Fix corrupt object panic caused by copy propagation

Mutating term values used as object keys (or set elements) is not safe
because the underlying hashtables get corrupted. This a known issue
that was previously fixed in the compiler (#1125). The simplest
solution in this case is to use the ast.Transform helper instead of
ast.Walk so that a copy of object and set terms is made.

Fixes #1177

Signed-off-by: Torin Sandall <torinsandall@gmail.com>