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_resolve/src/late.rs

@@ -586,7 +586,8 @@ impl<'a: 'ast, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
                     // Allow all following defaults to refer to this type parameter.
                     default_ban_rib.bindings.remove(&Ident::with_dummy_span(param.ident.name));
                 }
-                GenericParamKind::Const { ref ty, kw_span: _ } => {
+                GenericParamKind::Const { ref ty, kw_span: _, default: _ } => {
+                    // FIXME(const_generics_defaults): handle `default` value here
                     for bound in &param.bounds {
                         self.visit_param_bound(bound);
                     }