This commit aligns the `fetch` API and the `Request` / `Response`
classes belonging to it to the spec. This commit enables all the
relevant `fetch` WPT tests. Spec compliance is now at around 90%.
Performance is essentially identical now (within 1% of 1.9.0).
This commit introduces a performance optimization for the native HTTP
server. From my testing it is about 2-6% faster than `main`. Request
headers in the HTTP servers are now lazilly instatated when they are
accessed, rather than being preemptively wrapped in the `Headers` class.
