snippets: add rust snippets

Signed-off-by: Seongmin Lee <[e-mail]>

lua/snippets/rust.lua

···
     s("cl", {
         t"|", i(1), t"|", t" {", i(2), t"}",
     }),
+    s("fn", fmt([[
+        fn {}({}){} {
+        {}{}
+        }
+    ]], { i(1), i(2), i(3), t("\t"), i(4) })),
+    s("afn", fmt([[
+        async fn {}({}){} {
+        {}{}
+        }
+    ]], { i(1), i(2), i(3), t("\t"), i(4) })),
 })