bump minimum Rust to 1.53

1c9a556 started depending on arrays implementing IntoIterator. I keep
forgetting that 1.52 doesn't support this. There's really no compelling
reason to keep building on 1.52, so don't.

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     name: Rust ${{ matrix.rust }}
     strategy:
       matrix:
-        rust: [ "stable", "1.52", "nightly" ]
+        rust: [ "stable", "1.53", "nightly" ]
         include:
           - rust: nightly
             extra_args: "--features nightly --benches"

guide/build.md

@@ -216,7 +216,7 @@ following command:
 $ brew install ffmpeg node
 ```
 
-Next, you need Rust 1.52+ and Cargo. The easiest way to install them is by
+Next, you need Rust 1.53+ and Cargo. The easiest way to install them is by
 following the instructions at [rustup.rs](https://www.rustup.rs/). Avoid
 your Linux distribution's Rust packages, which tend to be too old.
 (At least on Debian-based systems; Arch and Gentoo might be okay.)