rust: make fns/pfns like the rest of the function templates (fix #323)

snippets/rust-mode/fns

@@ -1,5 +1,7 @@
 # -*- mode: snippet -*-
-# name: fn name(&self) -> Type;
+# name: fn name(&self) -> Type  { ... }
 # key: fns
 # --
-fn ${1:name}(${2:&self}) -> ${3:Type};
+fn ${1:name}(${2:&self}) -> ${3:Type}  {
+    $0
+}

snippets/rust-mode/pfns

@@ -1,5 +1,7 @@
 # -*- mode: snippet -*-
-# name: pub fn name(&self) -> Type;
+# name: pub fn name(&self) -> Type  { ... }
 # key: pfns
 # --
-pub fn ${1:name}(${2:&self}) -> ${3:Type};
+pub fn ${1:name}(${2:&self}) -> ${3:Type} {
+    $0
+}