stubgen: unify C extension and pure python stub generators with object oriented design #10658

Job	Run time
Test mypyc suite with 32-bit Python	10m 22s
Test suite with py38-ubuntu, mypyc-compiled	1m 43s
Test suite with py38-windows-64	36m 34s
Test suite with py39-ubuntu	26m 53s
Test suite with py310-ubuntu	21m 16s
Test suite with py311-ubuntu, mypyc-compiled	1m 9s
Test suite with py312-ubuntu, mypyc-compiled	1m 40s
mypyc runtime tests with py38-macos	7m 45s
mypyc runtime tests with py38-debug-build-ubuntu	8m 45s
Type check our own code (py38-ubuntu)	2m 24s
Type check our own code (py38-windows-64)	2m 42s
Formatting and code style with Black + ruff	55s
	2h 2m 8s

Provide feedback