Pass an encoding label to StreamingDecoder instead of requiring
callers to pre-resolve a Decoder reference.
This removes redundant decoder_for() plumbing from callers and lets
StreamingDecoder hide how chunked decoder state is represented.
Introduce IncrementalDocumentParser, which streams the response body
through a TextCodec::StreamingDecoder into the HTMLTokenizer one chunk
at a time. The tokenizer pauses when it runs out of input and resumes
once the next chunk is appended; when the body closes we close the
tokenizer's input stream so it can finish the parse.
DocumentLoading routes HTML responses through the new parser instead of
buffering the full body before handing it to HTMLParser.
