Update rustfmt to be consistent with setup in parity-common

fordN · fordN · commit aaadcf0505c0 · 2020-01-13T13:53:02.000-08:00
diff --git a/rustfmt.toml b/rustfmt.toml
@@ -1,2 +1,4 @@
 hard_tabs = true
-use_small_heuristics = 'Off'
+max_width = 120
+use_small_heuristics = "Max"
+edition = "2018"
