Add extra::arena::Arena::new{, _with_size}.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #8183 : omasanori/rust/migrate-new, r=sanxiyn

It seems that relatively new code uses `Foo::new()` instead of `Foo()` so I wrote a patch to migrate some structs to the former style.
Is it a right direction? If there are any guidelines not to use new()-style, could you add them to the [style guide](https://github.com/omasanori/rust/wiki/Note-style-guide)?