topdown: Fix bug in var rewriting during trace formatting

This commit fixes a bug in the var rewritting improvement that was
introduced in v0.16.0. With this change the rewriting is performed on
the deep copied node as it should be. Also, this change updates the
rewriting to use the ast.TransformVars helper instead of the generic
visitor so that object keys and sets are handled properly.

Fixes #2022

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