Initial Eio port

This switches capnp-rpc from Lwt to Eio. One particularly nice side
effect of this is that `Service.return_lwt` has gone, as there is no
distinction now between concurrent and non-concurrent service methods.

Notes:

- In this commit, everything is still using the "lwt" names to make the diff useful.
  In a future commit, this should be renamed. Also, some of the "unix"
  functions can be moved into the core library with Eio.
  This would likely be a good time to rename `capnp_rpc` to
  `capnp_rpc_protocol` or something, leaving the short name free for the
  main library.
- Mirage support is gone. Ideally, the regular library should work with Mirage 5.

Makefile

@@ -3,7 +3,7 @@
 default: test build-fuzz
 
 all:
-	dune build @install test/test.exe test-lwt/test_lwt.exe test-bin/calc.exe test-mirage/test_mirage.exe
+	dune build @install test/test.exe test-lwt/test_lwt.exe test-bin/calc.exe
 	rm -rf _build/_tests
 	dune runtest --no-buffer -j 1
 
@@ -19,7 +19,7 @@ clean:
 
 test:
 	rm -rf _build/_tests
-	dune build test/test.exe test-lwt/test_lwt.exe test-bin/calc.exe test-mirage/test_mirage.exe test-bin/echo/echo_bench.exe @install
+	dune build test/test.exe test-lwt/test_lwt.exe test-bin/calc.exe test-bin/echo/echo_bench.exe @install
 	#./_build/default/test/test.bc test core -ev 36
 	#./_build/default/test-lwt/test.bc test lwt -ev 3
 	dune build @runtest --no-buffer -j 1