This commit makes wasm iserv take advantage of the Pipe refactoring by
passing binary receiver/sender js callbacks from the dyld script. This
paves the way for piping the binary messages through WebSockets in
order to run wasm iserv in the browser, but more importantly, it
allows us to get rid of a horrible hack in the dyld script: we no
longer have to fake magical wasi file descriptors that are backed by
nodejs blocking I/O! The legacy hack was due to these facts:

- iserv only supported exchanging binary messages via handles backed
  by file descriptors
- In wasi you can't access host file descriptors passed by host ghc
- The nodejs wasi implementation only allows mapping host directories
  into the wasi vfs, not host file descriptors
- Named pipes with file paths (mkfifo) doesn't work well with nodejs
  wasi implementation, causes spurious testsuite failures on macos

But starting from this point, we can fully take advantage of
non-blocking I/O on the js side.

(cherry picked from commit a2103fd2)