rust: max_width=100, other rustfmt settings. #27071

These are the 12 stable and documented configuration options,
set to their default values.

use_small_heuristics is only stabilized in rustfmt 0.9, so maintain
support for 0.8.x for now by commenting it out.

comment_width is unstable and did nothing, since wrap_comments defaults
to false.

Default values gotten from `rustfmt --print-config default rustfmt.toml`.

https://github.com/rust-lang-nursery/rustfmt/blob/e7932fa9c2591c45a37a24305de90cb63128afcf/Configurations.md

src/rust/.rustfmt.toml

@@ -1,2 +1,12 @@
-max_width = 80
-comment_width = 80
+max_width = 100
+hard_tabs = false
+tab_spaces = 4
+newline_style = "Unix"
+#use_small_heuristics = "Default"
+reorder_imports = true
+reorder_modules = true
+remove_nested_parens = true
+merge_derives = true
+use_try_shorthand = false
+use_field_init_shorthand = false
+force_explicit_abi = true