feat: remove redundant nested momento module (momentohq#93)

Before this commit, the auth module was in a nested momento module. So,
to access it, you would need to look in `momento::momento::auth`. This
is, frankly, weird and unnecessary. This commit removes the nested
momento module and drops the auth module into the crate root.

src/momento/auth.rs → src/auth.rs

@@ -12,7 +12,7 @@ pub type LoginActionConsumer = fn(LoginAction) -> EarlyOutActionResult;
 /// Initiate a login workflow
 /// You need to provide an implementation for the LoginActions.
 /// ```rust
-/// # use momento::momento::auth::{login, LoginAction, LoginResult};
+/// # use momento::auth::{login, LoginAction, LoginResult};
 /// # async {
 ///     let result = login(|action| {
 ///       match action {

src/lib.rs

@@ -1,4 +1,4 @@
-pub mod momento;
+pub mod auth;
 pub mod response;
 pub mod simple_cache_client;