Advance pos_of_buffer_start when writing directly to a writer #2044

Job	Run time
linux nightly (all features)	2m 1s
linux beta (default features)	2m 22s
linux stable (default features)	2m 35s
Check sync-readme	38s
linux stable (with-bytes)	1m 13s
mega-linter	2m 11s
Miri test	5m 2s
rustfmt check	45s
CI self-check	9s
windows stable (default features)	3m 50s
	20m 46s

Provide feedback