This PR configures the formatting with a few defaults that aid
readability, in my opinion. One drawback of adding this file is that
rustfmt uses unstable features, now, and you have to use nightly to run
it:
```sh
cargo +nightly fmt --all
```
rust-lang/rust#82069 made error message that stem macro invocations more
verbose. Since Rust 1.54 (currently in beta) the message includes the
name of the offending macro.
This PR uses version_check to select the appropriate expected error
message.
