Refactor: Add write method to PgStream and BufStream #15

Job	Run time
clippy (sqlx-rt-oldapi) • runtime-tokio-rustls	31s
clippy (sqlx-rt-oldapi) • runtime-async-std-rustls	1m 4s
clippy (sqlx-rt-oldapi) • runtime-async-std-native-tls	44s
clippy (sqlx-oldapi) • runtime-tokio-rustls,postgres,macros	2m 23s
clippy (sqlx-core-oldapi) • postgres • runtime-tokio-rustls	1m 15s
clippy (sqlx-rt-oldapi) • runtime-tokio-native-tls	34s
clippy (sqlx-core-oldapi) • sqlite • runtime-tokio-rustls	1m 19s
clippy (sqlx-oldapi) • runtime-async-std-rustls,postgres,macros	2m 28s
clippy (sqlx-core-oldapi) • mssql • runtime-tokio-rustls	1m 21s
clippy (sqlx-oldapi) • runtime-tokio-rustls,mysql,macros	2m 24s
clippy (sqlx-oldapi) • runtime-tokio-rustls,sqlite,macros	2m 24s
clippy (sqlx-core-oldapi) • any • runtime-tokio-rustls	1m 15s
clippy (sqlx-core-oldapi) • mysql • runtime-tokio-rustls	1m 14s
	18m 56s

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