Add an experimental feature gate for function delegation

In accordance with the [process](https://github.com/rust-lang/lang-team/blob/master/src/how_to/experiment.md).

Detailed description of the feature can be found in the RFC repo - rust-lang/rfcs#3530.

TODO: find a lang team lang-team liaison.

compiler/rustc_feature/src/unstable.rs

@@ -456,6 +456,8 @@ declare_features! (
     (unstable, ffi_returns_twice, "1.34.0", Some(58314), None),
     /// Allows using `#[repr(align(...))]` on function items
     (unstable, fn_align, "1.53.0", Some(82232), None),
+    /// Support delegating implementation of functions to other already implemented functions.
+    (incomplete, fn_delegation, CURRENT_RUSTC_VERSION, None /*FIXME*/, None),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078), None),
     /// Infer generic args for both consts and types.