Separate contract feature gates for the internal machinery

The extended syntax for function signature that includes contract clauses
should never be user exposed versus the interface we want to ship
externally eventually.

compiler/rustc_span/src/symbol.rs

@@ -1716,6 +1716,8 @@ symbols! {
         rustc_const_stable,
         rustc_const_stable_indirect,
         rustc_const_unstable,
+        rustc_contracts,
+        rustc_contracts_internals,
         rustc_conversion_suggestion,
         rustc_deallocator,
         rustc_def_path,