switch lint config to manifest-lint feature

I removed some lint configuration in the process:

* `#[allow(clippy::suspicious_else_formatting)]` because nothing is
  currently triggering it.
* `#[warn(clippy::future_not_send)]` because some stuff under
  `src/lib.rs` is. And also like, auto-trait leakage generally means
  this isn't a problem, and if things really need to be `Send`, then
  you'll probably know to mark it manually.
* `#[warn(rust_2018_idioms)]` and replaced it with
  `explicit-outlives-requirements = "warn"` which is the most useful
  lint in that group that isn't enabled by default.

src/lib.rs

@@ -1,12 +1,3 @@
-#![warn(
-    rust_2018_idioms,
-    unused_qualifications,
-    clippy::cloned_instead_of_copied,
-    clippy::str_to_string
-)]
-#![allow(clippy::suspicious_else_formatting)]
-#![deny(clippy::dbg_macro)]
-
 pub mod api;
 mod config;
 mod database;