rustfmt: Set use_small_heuristics to Max

As we do in `rust-bitcoin` set the `use_small_heuristics` option to max,
this sets all granular width options to the same as the max value (100).

Only change the `rustfmt` config file, we will run the formatter as a
separate patch.

Author: tcharding

rustfmt.toml

@@ -1,16 +1,20 @@
-max_width = 100
 hard_tabs = false
 tab_spaces = 4
 newline_style = "Auto"
 indent_style = "Block"
-use_small_heuristics = "Default"
-fn_call_width = 60
-attr_fn_like_width = 70
-struct_lit_width = 18
-struct_variant_width = 35
-array_width = 60
-chain_width = 60
-single_line_if_else_max_width = 50
+
+max_width = 100 # This is number of characters.
+# `use_small_heuristics` is ignored if the granular width config values are explicitly set.
+use_small_heuristics = "Max"    # "Max" == All granular width settings same as `max_width`.
+# # Granular width configuration settings. These are percentages of `max_width`.
+# fn_call_width = 60
+# attr_fn_like_width = 70
+# struct_lit_width = 18
+# struct_variant_width = 35
+# array_width = 60
+# chain_width = 60
+# single_line_if_else_max_width = 50
+
 wrap_comments = false
 format_code_in_doc_comments = false
 comment_width = 80