first pass at default values for const generics

- Adds optional default values to const generic parameters in the AST
  and HIR
- Parses these optional default values
- Adds a `const_generics_defaults` feature gate

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -145,7 +145,8 @@ fn inject_impl_of_structural_trait(
                 *default = None;
                 ast::GenericArg::Type(cx.ty_ident(span, param.ident))
             }
-            ast::GenericParamKind::Const { ty: _, kw_span: _ } => {
+            ast::GenericParamKind::Const { ty: _, kw_span: _, default } => {
+                *default = None;
                 ast::GenericArg::Const(cx.const_ident(span, param.ident))
             }
         })