Speed up finding function type variables (#16562) #417

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	19m 29s
Test mypyc suite with 32-bit Python	8m 11s
Test suite with py38-windows-64	26m 5s
Test suite with py39-ubuntu	19m 7s
Test suite with py310-ubuntu	17m 35s
Test suite with py311-ubuntu, mypyc-compiled	17m 6s
Test suite with py312-ubuntu, mypyc-compiled	19m 45s
mypyc runtime tests with py39-macos	5m 35s
mypyc runtime tests with py38-debug-build-ubuntu	5m 21s
Type check our own code (py38-ubuntu)	1m 46s
Type check our own code (py38-windows-64)	2m 34s
Formatting and code style with Black + ruff	33s
	2h 23m 7s

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