Create a dedicated workflow type for creating logins.

Nasty design corner. Logins need to be created in three places:

1. In tests, using app.logins().create(…);
2. On initial setup, using app.setup().initial(…); and
3. When accepting invites, using app.invites().accept(…).

These three places do the same thing with respect to logins, but also do a varying mix of other things. Testing is the simplest and _only_ creates a login. Initial setup and invite acceptance both issue a token for the newly-created login. Accepting an invite also invalidates the invite. Previously, those three functions have been copy-pasted variations on a theme. Now that we have validation, the copy-paste approach is no longer tenable; it will become increasingly hard to ensure that the three functions (plus any future functions) remain in synch.

To accommodate the variations while consolidating login creation, I've added a typestate-based state machine, which is driven by method calls:

* A creation attempt begins with `let create = Create::begin()`. This always succeeds; it packages up arguments used in later steps, but does nothing else.
* A creation attempt can be validated using `let validated = create.validate()?`. This may fail. Input validation and password hashing are carried out at this stage, making it potentially expensive.
* A validated attempt can be stored in the DB, using `let stored = validated.store(&mut tx).await?`. This may fail. The login will be written to the DB; the caller is responsible for transaction demarcation, to allow other things to take place in the same transaction.
* A fully-stored attempt can be used to publish events, using `let login = stored.publish(self.events)`. This always succeeds, and unwraps the state machine to its final product (a `login::History`).

1 files changed, 2 insertions, 1 deletions

diff --git a/src/login/mod.rs b/src/login/mod.rs
index 6d10e17..5a6d715 100644
--- a/src/login/mod.rs
+++ b/src/login/mod.rs
@@ -1,4 +1,5 @@
 pub mod app;
+pub mod create;
 pub mod event;
 mod history;
 mod id;
@@ -6,7 +7,7 @@ pub mod password;
 pub mod repo;
 mod routes;
 mod snapshot;
-pub mod validate;
+mod validate;
 
 pub use self::{
     event::Event, history::History, id::Id, password::Password, routes::router, snapshot::Login,