Stricter body types, etags for binary responses

[Rich-Harris]

supersedes #1381, fixes #1136. Updates request/response body types, and generates etags for binary bodies

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[drzax]

These stricter body types appear to prevent serving image binaries with the correct content-type. Unless I've missed something, or misunderstood how they should be served. Previously this worked, but now serves a 500 because Buffer is an instance of Uint8Array.

    return {
      headers: {
        'Content-Type': `image/png`
      },
      body: bufferContainingImageBinary
    };