Correctly handle PartialTypeVars in inference #2713

Job	Run time
Performance (hand_written)	7m 32s
Performance (derived)	4m 54s
Performance (inter-AD)	7m 14s
basic-1-x64	10m 54s
rrules-1-x64	24m 45s
ext/differentiation_interface-1-x64	6m 34s
integration_testing/misc-1-x64	12m 21s
integration_testing/misc_abstract_array-1-x64	13m 52s
integration_testing/diff_tests-1-x64	5m 54s
integration_testing/distributions-1-x64	5m 49s
integration_testing/gp-1-x64	16m 19s
integration_testing/array-1-x64	12m 1s
integration_testing/turing-1-x64	5m 58s
integration_testing/temporalgps-1-x64	3m 10s
basic-1.10-x64	9m 21s
integration_testing/turing-1.10-x64	4m 59s
basic-1.10-x86	10m 8s
	2h 41m 45s

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