Add `unit_separator` to `#humanize` and `#humanize_bytes` #4381

Job	Run time
Build interpreter	9m 27s
Test Interpreter	4m 4s
Test primitives_spec with interpreter	29s
Test std_spec with interpreter (0)	2m 55s
Test std_spec with interpreter (1)	3m 9s
Test std_spec with interpreter (2)	3m 7s
Test std_spec with interpreter (3)	2m 9s
	25m 20s

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