Add benchmark for account creation

benches/create_account.rs

@@ -0,0 +1,26 @@
+use async_std::path::PathBuf;
+use async_std::task::block_on;
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use deltachat::accounts::Accounts;
+use tempfile::tempdir;
+
+async fn create_accounts(n: u32) {
+    let dir = tempdir().unwrap();
+    let p: PathBuf = dir.path().join("accounts").into();
+
+    let accounts = Accounts::new("my_os".into(), p.clone()).await.unwrap();
+
+    for expected_id in 2..n {
+        let id = accounts.add_account().await.unwrap();
+        assert_eq!(id, expected_id);
+    }
+}
+
+fn criterion_benchmark(c: &mut Criterion) {
+    c.bench_function("create 1 account", |b| {
+        b.iter(|| block_on(async { create_accounts(black_box(1)).await }))
+    });
+}
+
+criterion_group!(benches, criterion_benchmark);
+criterion_main!(benches);