fetch: limit web streams usage in body mixin methods

The fetch spec requires us to create a new web Readable stream for every Response and Request object, which can likely be avoided in most cases.

Given the following examples:

new Response('abc')
new Response(new TextEncoder().encode('abc'))
new Response(new Blob(['abc']))
// etc.

A web stream is created and then converted back to a string (or blob, arraybuffer, etc.)! The following diff improves .text() performance when the body passed is a string by 10%, as a simple demo (we could further limit web stream usage for more performance gains!!!).

diff --git a/lib/fetch/response.js b/lib/fetch/response.js
index 1029dbef..c2d2bd3d 100644
--- a/lib/fetch/response.js
+++ b/lib/fetch/response.js
@@ -154,7 +154,7 @@ class Response {
     // 4. If body is non-null, then set bodyWithType to the result of extracting body.
     if (body != null) {
       const [extractedBody, type] = extractBody(body)
-      bodyWithType = { body: extractedBody, type }
+      bodyWithType = { body: extractedBody, type, source: body }
     }

     // 5. Perform initialize a response given this, init, and bodyWithType.
diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index 400687ba..9e4302a9 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -824,6 +824,11 @@ function fullyReadBody (body, processBody, processBodyError) {
   //    with taskDestination.
   const errorSteps = (error) => queueMicrotask(() => processBodyError(error))

+  if (typeof body.source === 'string') {
+    successSteps(new TextEncoder().encode(body.source))
+    return
+  }
+
   // 4. Let reader be the result of getting a reader for body’s stream.
   //    If that threw an exception, then run errorSteps with that
   //    exception and return.

cc @anonrig