Additionally, if the existing `Request`'s body is disturbed, the Request creation
should fail.
This change also updates the step numbers in the Request constructor to match
whatwg/fetch#1249.
This commit removes all JS based text encoding / text decoding. Instead
encoding now happens in Rust via encoding_rs (already in tree). This
implementation retains stream support, but adds the last missing
encodings. We are incredibly close to 100% WPT on text encoding now.
This should reduce our baseline heap by quite a bit.
