Turns out HashTable::contains() doesn't solely use hash() for equality
checks, so the lack of a proper equals() implementation broke the check
in convert_header_names_to_a_sorted_lowercase_set() and caused duplicate
entries in header_names_set.
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.
