Recommend .cargo/config.toml over .cargo/config (#459)

As of Rust version 1.39.0, `config.toml` is the preferred filename for
`cargo`s configuration file. Incidentally added a newline at EOF.

https://doc.rust-lang.org/cargo/reference/config.html

.cargo/config_fast_builds

@@ -1,4 +1,4 @@
-# Rename this file to `config` to enable "fast build" configuration. Please read the notes below.
+# Rename this file to `config.toml` to enable "fast build" configuration. Please read the notes below.
 
 # NOTE: For maximum performance, build using a nightly compiler
 # If you are using rust stable, remove the "-Zshare-generics=y" below.
@@ -19,4 +19,4 @@ rustflags = ["-Zshare-generics=y"]
 # Optional: Uncommenting the following improves compile times, but reduces the amount of debug info to 'line number tables only'
 # In most cases the gains are negligible, but if you are on macos and have slow compile times you should see significant gains.
 #[profile.dev]
-#debug = 1
+#debug = 1