_decimal: Store a module state in objects for faster access

[neonene]

Feature or enhancement

Proposal:

It is known to be efficient to get a module state from an object directly rather than from a type. The same can go for the _decimal's context object (i.e. extending the PyDecContextObject struct).

Extending PyDecObject, which made no perf difference for me, seems to be a waste of memory with many objects.

cc @vstinner @erlend-aasland @mdboom

Links to previous discussion of this feature:

• PyType_GetModuleByDef is really slow. #119663

Linked PRs

• gh-124688: _decimal: Get a module state from ctx objects for performance #124691

[picnixz]

Closing since completed in dc12237.