Rollup merge of #110766 - m-ou-se:fmt-rt, r=jyn514

More core::fmt::rt cleanup.

- Removes the `V1` suffix from the `Argument` and `Flag` types.

- Moves more of the format_args lang items into the `core::fmt::rt` module. (The only remaining lang item in `core::fmt` is `Arguments` itself, which is a public type.)

Part of https://github.com/rust-lang/rust/issues/99012

Follow-up to https://github.com/rust-lang/rust/pull/110616

compiler/rustc_span/src/symbol.rs

@@ -129,8 +129,7 @@ symbols! {
         Any,
         Arc,
         Argument,
-        ArgumentV1,
-        ArgumentV1Methods,
+        ArgumentMethods,
         Arguments,
         AsMut,
         AsRef,