The spec declares these as a byte sequence, which we then implemented as
a ByteBuffer. This has become pretty awkward to deal with, as evidenced
by the plethora of `MUST(ByteBuffer::copy(...))` and `.bytes()` calls
everywhere inside Fetch. We would then treat the bytes as a string
anyways by wrapping them in StringView everywhere.
We now store these as a ByteString. This is more comfortable to deal
with, and we no longer need to continually copy underlying storage (as
ByteString is ref-counted).
This work is largely preparatory for an upcoming HTTP header refactor.
Generally just define things in the order they are declared (will make a
change to use ByteString in this file a bit easier to follow). Also make
a couple of free functions be class methods on Header / HeaderList.
Disallow calling `StringBase::bytes()` on temporaries to avoid returning
`ReadonlyBytes` that outlive the underlying string.
With this change, we catch a real UAF:
`load_result.data = maybe_response.release_value().bytes();`
All other updated call sites were already safe, they just needed to use
an intermediate named variable to satisfy the new lvalue-only
requirement.
