Initial work on pipelines

faststreams/async_backend.nim

@@ -0,0 +1,31 @@
+const
+  faststreams_async_backend {.strdefine.} = "chronos"
+
+when faststreams_async_backend == "chronos":
+  import chronos # import chronos/[asyncfutures2, asyncmacro2]
+  export chronos # export asyncfutures2, asyncmacro2
+
+  template faststreamsAwait*(f: Future): untyped =
+    await f
+
+elif faststreams_async_backend in ["std", "asyncdispatch"]:
+  import std/[asyncfutures, asyncmacro]
+  export asyncfutures, asyncmacro
+
+  template faststreamsAwait*(awaited: Future[T]): untyped =
+    # TODO revisit after https://github.com/nim-lang/Nim/pull/12085/ is merged
+    let f = awaited
+    yield f
+    if not isNil(f.error):
+      raise f.error
+    f.read
+
+else:
+  {.fatal: "Unrecognized network backend: " & faststreams_async_backend.}
+
+template raiseFaststreamsError*(errMsg: string, body: untyped) =
+  try:
+    body
+  except CatchableError as err:
+    raise newException(IOError, errMsg, err)
+