Store offline data in one file per query / remove sqlx-data.json

[jplatte]

Query data is now stored in .sqlx/{query_hash}.json directly by the macro invocations, rather than first writing to target/sqlx/{input_span_hash}.json and then collecting those into sqlx-data.json separately.

• save offline data (if .sqlx exists)
• load offline data
• create .sqlx when running cargo sqlx prepare
• diff target/sqlx with .sqlx in cargo sqlx prepare --check
• add / update tests
• update documentation

Closes #570.