* WalkMapper: fix map_einsum

* Update transform.py

This reverts commit 6526f804.

This reverts commit 87c793c2.

Add TopoSortMapper

show addr of node in visualization

Introduced UpstreamLoopyContext that only denotes the context for
upstream expressions

* make pt.(Placeholder|SizeParam) equality comparable

The only input argument for which it makes sense to have identity comparison is
data wrappers to that the DAG comparisons are efficient. This also led to some
downstream re-writing of mappers's cache keys that relied on object equality.

* Placeholder without a name is moot

No sensible program would actually needed a non-named placeholder.

* fixup! make pt.(Placeholder|SizeParam) equality comparable

* adds a note about DataWrapper.__eq__'s implementation

* fixup! make pt.(Placeholder|SizeParam) equality comparable

* Clarify DataWrapper equality definition

* Clarify comment explaining use of id() for CachedWalkMapper

Co-authored-by: Andreas Klöckner <inform@tiker.net>