Improve wasm32-wasi support

[Starting from 3.41.0 SQLite has wasm32-wasi support out of the
box.][0]

- Set `-DSQLITE_THREADSAFE=0`. Fixes:

```
$ wasmtime target/wasm32-wasi/release/examples/persons.wasm
Error: failed to run main module `target/wasm32-wasi/release/examples/persons.wasm`

Caused by:
    0: failed to instantiate "target/wasm32-wasi/release/examples/persons.wasm"
    1: unknown import: `env::pthread_mutexattr_init` has not been defined
```

- Drop `-DSQLITE_OS_OTHER`. Fixes:

```
$ wasmtime target/wasm32-wasi/release/examples/persons.wasm
Error: failed to run main module `target/wasm32-wasi/release/examples/persons.wasm`

Caused by:
    0: failed to instantiate "target/wasm32-wasi/release/examples/persons.wasm"
    1: unknown import: `env::sqlite3_os_init` has not been defined
```

- [Add wasi specific build flags][1]
- Add basic example
- Also, add instructions how to run it against wasm32-wasi.

Using of file databases is also working, though `--mapdir` arg
should be provided to `wasmtime run`.

[0]: https://wasmlabs.dev/articles/sqlite-wasi-support/
[1]: https://github.com/vmware-labs/webassembly-language-runtimes/blob/main/libs/sqlite/wasi-patches/wlr-build.sh#L11

examples/persons/main.rs

@@ -0,0 +1,42 @@
+extern crate rusqlite;
+use rusqlite::{Connection, Result};
+
+struct Person {
+    id: i32,
+    name: String,
+}
+fn main() -> Result<()> {
+    let conn = Connection::open_in_memory()?;
+
+    conn.execute(
+        "CREATE TABLE IF NOT EXISTS persons (
+            id INTEGER PRIMARY KEY,
+            name TEXT NOT NULL
+        )",
+        (), // empty list of parameters.
+    )?;
+
+    conn.execute(
+        "INSERT INTO persons (name) VALUES (?1), (?2), (?3)",
+        ["Steven", "John", "Alex"].map(|n| n.to_string()),
+    )?;
+
+    let mut stmt = conn.prepare("SELECT id, name FROM persons")?;
+    let rows = stmt.query_map([], |row| {
+        Ok(Person {
+            id: row.get(0)?,
+            name: row.get(1)?,
+        })
+    })?;
+
+    println!("Found persons:");
+
+    for person in rows {
+        match person {
+            Ok(p) => println!("ID: {}, Name: {}", p.id, p.name),
+            Err(e) => eprintln!("Error: {:?}", e),
+        }
+    }
+
+    Ok(())
+}