Advance pos_of_buffer_start when writing directly to a writer #2027

Job	Run time
linux beta (default features)	2m 29s
linux stable (with-bytes)	1m 23s
linux nightly (all features)	1m 55s
linux stable (default features)	2m 22s
mega-linter	2m 11s
Check sync-readme	36s
Miri test	5m 3s
rustfmt check	49s
windows stable (default features)	2m 46s
CI self-check	11s
	19m 45s

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