This wires up checkpointing to the SQLite C API. We don't respect the
checkpointing mode because core does not have that nor do we report back
some stats.
Refs: #478
This includes an inner struct in Page wrapped with Unsafe cell to access
it. This is done intentionally because concurrency control of pages is
handled by pages and not by the page itself.
Add enough stubs to be able to link CPython build against Limbo's SQLite
compatibility API:
```
export LIBSQLITE3_LIBS=../limbo/target/debug/liblimbo_sqlite3.a
./configure
make -j8
[snip]
gcc -shared Modules/_sqlite/blob.o Modules/_sqlite/connection.o Modules/_sqlite/cursor.o Modules/_sqlite/microprotocols.o Modules/_sqlite/module.o Modules/_sqlite/prepare_protocol.o Modules/_sqlite/row.o Modules/_sqlite/statement.o Modules/_sqlite/util.o ../limbo/target/debug/liblimbo_sqlite3.a -o Modules/_sqlite3.cpython-314-x86_64-linux-gnu.so
thread '<unnamed>' panicked at sqlite3/src/lib.rs:751:5:
not yet implemented
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5
make: *** [Makefile:1511: checksharedmods] Aborted (core dumped)
```
All that's left is to actually implement all of this...
This adds initial SQLite C ABI compatibility to Limbo to make sure we
drive the Rust API in the right way that allows us to implement SQLite
semantics.
