Implement shared memory WAL. (#71)

- enabled by default on 64-bit macOS and Linux (`amd64`/`arm64`)
- depends on merged but unreleased wazero
- may cause small performance regression
- users may need WithMemoryLimitPages if not enough address space available
- needs docs

sqlite.go

@@ -85,7 +85,7 @@ func instantiateSQLite() (sqlt *sqlite, err error) {
 	}
 
 	sqlt = new(sqlite)
-	sqlt.ctx = util.NewContext(context.Background())
+	sqlt.ctx = util.NewContext(context.Background(), vfs.SupportsSharedMemory)
 
 	sqlt.mod, err = instance.runtime.InstantiateModule(sqlt.ctx,
 		instance.compiled, wazero.NewModuleConfig())