Running `cargo fmt` or `cargo +nightly fmt` gives the following
warnings:
```
Warning: Unknown configuration option `fn_brace_style`
Warning: Unknown configuration option `item_brace_style`
Warning: Unknown configuration option `struct_lit_multiline_style`
Warning: Unknown configuration option `where_trailing_comma`
```
rustfmt changed these options here, and this PR reflects the changes:
* `fn_brace_style` and `item_brace_style` were folded into
`brace_style`: f8074b340d (diff-6d8a5e2104954a026de2bfe2b936aea4R541)
* `struct_lit_multiline_style` option was renamed to
`struct_lit_single_line`: 45d4f7a2dd (diff-9de0a570da3404ffd4b364ef6a9f8195L1749)
* `where_trailing_comma` was removed, but appears to have no
replacement: 6a58d91239 (diff-6d8a5e2104954a026de2bfe2b936aea4L369)
So I left it in.
Running `cargo fmt` over the bindgen codebase produces a _lot_ of
changes, so I haven't done that. But I think it'd be a good idea to do
it at some point: opening PRs against a project that has its own
rustfmt.toml file but doesn't seem to follow it is a confusing process.
