Handle Struct { box i } syntax

Named structs can have `box` patterns that will bind to their fields.
This is similar to the behavior of the `ref` and `mut` fields, but is at
least a little bit surprising.

crates/ra_parser/src/grammar/patterns.rs

@@ -161,6 +161,9 @@ fn record_field_pat_list(p: &mut Parser) {
             T![..] => p.bump(),
             IDENT if p.nth(1) == T![:] => record_field_pat(p),
             T!['{'] => error_block(p, "expected ident"),
+            T![box] => {
+                box_pat(p);
+            }
             _ => {
                 bind_pat(p, false);
             }