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_hir/src/hir.rs

@@ -418,6 +418,8 @@ pub enum GenericParamKind<'hir> {
     },
     Const {
         ty: &'hir Ty<'hir>,
+        /// Optional default value for the const generic param
+        default: Option<AnonConst>,
     },
 }
 

compiler/rustc_hir/src/intravisit.rs

@@ -877,7 +877,12 @@ pub fn walk_generic_param<'v, V: Visitor<'v>>(visitor: &mut V, param: &'v Generi
     match param.kind {
         GenericParamKind::Lifetime { .. } => {}
         GenericParamKind::Type { ref default, .. } => walk_list!(visitor, visit_ty, default),
-        GenericParamKind::Const { ref ty } => visitor.visit_ty(ty),
+        GenericParamKind::Const { ref ty, ref default } => {
+            visitor.visit_ty(ty);
+            if let Some(ref default) = default {
+                visitor.visit_anon_const(default);
+            }
+        }
     }
     walk_list!(visitor, visit_param_bound, param.bounds);
 }