syntax: Add ToTokens for Option<T>
syntax: Add ToTokens for Attribute_
syntax: Generalize ToTokens impl for Vec<T>

It will now `flat_map` over the elements of a `Vec<T>` if
`T: ToTokens`
syntax: Add quote_method!
syntax: Add ToTokens impl for Method
auto merge of #15732 : bgamari/rust/to-tokens, r=alexcrichton

Here I add a `ToTokens` impl for `Attribute_` and `Option<T>`, as well as generalize the impl for `Vec<T>`