Bump version to 1.9.0

[quinnj]

Merge after #361 and #364. Then comment on the merge commit:

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Release notes:

## New features
- `SQLite.Blob`: read and overwrite existing BLOB/TEXT values incrementally as a seekable `IO` stream (#361, closes #345).
- `DBInterface.executemultiple` now runs every statement in the SQL string and returns each result (#364).

## Behavior changes
- `DBInterface.execute`, `DBInterface.prepare` and `SQLite.execute` now throw `ArgumentError` for SQL with more than one statement. Before, only the first statement ran and the rest were silently ignored (#364, closes #303). Use `DBInterface.executemultiple` to run all of them.

## Fixes
- `close(db)` now finishes statements that were abandoned but not yet garbage-collected, so they can't hold locks or commit after close returns (#362).
- Aggregate UDFs no longer copy their whole state on every row. A growing-vector aggregate over 4,096 rows went from 38 ms to 1.3 ms (#363).
- Unsigned and other non-`Int64` integer parameters bind as SQLite integers, so `WHERE x = ?` with a `UInt16` matches (#356, closes #313).
- Starting a transaction keeps temporary tables (#357). UDF errors are contained, and aggregate state is released safely (#360).
- Docs: connection and statement cleanup, appending with `load!`, and saving an in-memory database with `SQLite.backup` (#350, #358, #359).

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