Remove the pipes compiler

The pipes compiler produced data types that encoded efficient and safe
bounded message passing protocols between two endpoints. It was also
capable of producing unbounded protocols.

It was useful research but was arguably done before its proper time.

I am removing it for the following reasons:

* In practice we used it only for producing the `oneshot` and `stream`
  unbounded protocols and all communication in Rust use those.
* The interface between the proto! macro and the standard library
  has a large surface area and was difficult to maintain through
  language and library changes.
* It is now written in an old dialect of Rust and generates code
  which would likely be considered non-idiomatic.
* Both the compiler and the runtime are difficult to understand,
  and likewise the relationship between the generated code and
  the library is hard to understand. Debugging is difficult.
* The new scheduler implements `stream` and `oneshot` by hand
  in a way that will be significantly easier to maintain.

This shouldn't be taken as an indication that 'channel protocols'
for Rust are not worth pursuing again in the future.

Concerned parties may include: @graydon, @pcwalton, @eholk, @bblum

The most likely candidates for closing are #7666, #3018, #3020, #7021, #7667, #7303, #3658, #3295.