Integrate with the streams standard by adding TextEncoderStream and
TextDecoderStream transform streams to the standard. These enable
binary<>text conversions on a ReadableStream using the `pipeThrough()`
method (see https://streams.spec.whatwg.org/#rs-pipe-through).

A TextEncoderStream object can be used to transform a stream of strings
to a stream of bytes in UTF-8 encoding. A TextDecoderStream object can
be used to transform a stream of bytes in the encoding passed to the
constructor to strings.

There is a prollyfill and tests for the new functionality at
https://github.com/GoogleChromeLabs/text-encode-transform-prollyfill.

Closes whatwg#72.

* Remove unnecessary link-defaults
* Make headers sentence case
* Linkify GenericTransformStream
* Indicate the GenericTransformStream's *transform* is a
  TransformStream.
* Add [INFRA] to the reference to the Infra Standard.

Remove the encoding, ignore BOM flag and error mode slots from
TextDecoder and TextDecoderStream and reference the versions in
TextDecoderAttributes instead.

Also remove the "transform" slot from TextDecoderStream and
TextEncoderStream and reference GenericTransformStream's version
instead.

Rename TextXcoderAlgorithms to TextXcoderCommon and move the "serialize
stream" from TextDecoder to TextDecoderCommon so that it can be shared
sanely with TextDecoderStream.

We no longer have explicit language rejecting detached buffers, however
the "get a copy of the buffer source" algorithm will throw exceptions
for them. Convert those exception to rejections to get the effect of the
check.

Also stop explicitly importing "transform stream" as it is now exported
from the streams standard.