Make pt.(Placeholder|SizeParam) equality comparable (#123)

* 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>