Support compiling libSQL to Wasm with virtual WAL

[psarna]

This PR contains a demo which uses Wasmtime runtime to run libSQL in a WebAssembly instance, compiled in WAL mode. Right now it's full of stubs, and a VFS that calls back to the host for each operation. Once these stubs are implemented and we also provide a virtual WAL implementation that calls back to the host, we'll have a solid foundation for running libSQL with replication as a Wasm instance.

Step 1: compile libsql.wasm (should be automated with build.rs, I'm just lazy)

cd libsql-sqlite3
make wasi

Step 2: run the demo

cd ext/libsql-wasi-demo
RUST_LOG=info,libsql_wasi_demo=trace cargo run

Step 3: profit

$ file /tmp/wasm-demo.db*
/tmp/wasm-demo.db:     SQLite 3.x database, last written using SQLite version 3044000, writer version 2, read version 2, file counter 1, database pages 1, cookie 0, schema 0, unknown 0 encoding, version-valid-for 1
/tmp/wasm-demo.db-wal: empty
$ sqlite3 /tmp/wasm-demo.db "pragma journal_mode"
wal

Virtual WAL is not implemented in this patch, but this demo already successfully used WAL mode from a WebAssembly instance and created a WAL.

Next steps (arbitrary order):

1. forge this demo into a library we can later use to run libsql Wasm instances for users
2. add virtual WAL support

[MarinPostma]

@psarna could we base that on WAL v2 😄 It'll never get merged if we keep adding stuff on top of v1 🙏

[MarinPostma]

ah wait it's actually only VFS

[psarna]

it's only a VFS stub for now, next week I'll fill all the implementation and then add virtual WAL too. But even regular WAL should work if the VFS is properly implemented, and that will be a major win: we'll have proof that libsql runs in WAL mode when compiled to Wasm.

[MarinPostma]

my point was more that I will need to port this code with wal v2, which means wal v2 is not getting merged any time soon 😢

[psarna]

Why? It isn't in any way intrusive with WAL, I'll just first implement it with what we have, and then port it to walv2 later, but it should be quite trivial (and nonblocking, worst case it breaks for a short while)

[psarna]

Breakthrough: the demo managed to use libsql compiled to Wasm in WAL mode, and store actual data in WAL. Next steps - make it a library, and add virtual WAL support.