Advance pos_of_buffer_start when writing directly to a writer #2015

Job	Run time
linux stable (default features)	7m 11s
Check sync-readme	45s
linux beta (default features)	7m 50s
linux nightly (all features)	7m 6s
linux stable (with-bytes)	6m 9s
mega-linter	2m 24s
Miri test	32s
rustfmt check	5m 38s
CI self-check	13s
windows stable (default features)	2m 42s
	40m 30s

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