Add a rustfmt.toml (#8982)

* Force push to clean up PR mess

* Update rustfmt.toml

Co-authored-by: Squirrel <gilescope@gmail.com>

* Run `cargo fmt` again with Giles' changes

* Unformat utils

Co-authored-by: Squirrel <gilescope@gmail.com>

substrate/rustfmt.toml

@@ -0,0 +1,22 @@
+# Basic
+hard_tabs = true
+max_width = 100
+use_small_heuristics = "Max"
+# Imports
+imports_granularity = "Crate"
+reorder_imports = true
+# Consistency
+newline_style = "Unix"
+normalize_comments = true
+normalize_doc_attributes = true
+# Misc
+chain_width = 80
+spaces_around_ranges = false
+binop_separator = "Back"
+reorder_impl_items = false
+match_arm_leading_pipes = "Preserve"
+match_arm_blocks = false
+match_block_trailing_comma = true
+trailing_comma = "Vertical"
+trailing_semicolon = false
+use_field_init_shorthand = true