This is needed to eventually share a header list between a Request or
Response object's internal infra request/response and the object's
exposed Header object.
A Request/Response instance should always be heap-allocated and have
clear ownership, so let's also wrap it in a NonnullOwnPtr instead of
putting them on the stack.
The Fetch spec unfortunately will cause a name clash between the Request
concept and the Request JS object - both cannot live in the Web::Fetch
namespace, and WrapperGenerator generally assumes `Web::<Name>` for
things living in the `<Name>/` subdirectory, so let's instead move infra
code into its own namespace - it already sits in a (sub-)subdirectory
anyway.
