Don't import/export chronos by default (#20)

Chronos support is optional and should not have to be imported in order
to use faststreams for non-async use cases, as doing so pollutes the
global namespace and slows down compilation.

`async` support must now explicitly be enabled with
-d:async_backend=chronos|asyncdispatch

faststreams.nimble

@@ -1,7 +1,7 @@
 mode = ScriptMode.Verbose
 
 packageName   = "faststreams"
-version       = "0.2.0"
+version       = "0.3.0"
 author        = "Status Research & Development GmbH"
 description   = "Nearly zero-overhead input/output streams for Nim"
 license       = "Apache License 2.0"
@@ -21,7 +21,12 @@ proc test(env, path: string) =
     lang = getEnv"TEST_LANG"
 
   exec "nim " & lang & " " & env &
-    " -r --hints:off --skipParentCfg " & path
+    " -d:async_backend=none -r --hints:off --skipParentCfg " & path
+  exec "nim " & lang & " " & env &
+    " -d:async_backend=chronos -r --hints:off --skipParentCfg " & path
+  # TODO std backend is broken / untested
+  # exec "nim " & lang & " " & env &
+  #  " -d:async_backend=chronos -r --hints:off --skipParentCfg " & path
 
 task test, "Run all tests":
   test "-d:debug   --threads:on", "tests/all_tests"

faststreams/async_backend.nim

@@ -1,14 +1,25 @@
 const
-  faststreams_async_backend {.strdefine.} = "chronos"
+  # To compile with async support, use `-d:async_backend=chronos|asyncdispatch`
+  async_backend {.strdefine.} = "none"
+
+const
+  faststreams_async_backend {.strdefine.} = ""
+
+when faststreams_async_backend != "":
+  {.fatal: "use `-d:async_backend` instead".}
 
 type
   CloseBehavior* = enum
     waitAsyncClose
     dontWaitAsyncClose
 
-const debugHelpers* = defined(debugHelpers)
+const
+  debugHelpers* = defined(debugHelpers)
+  fsAsyncSupport* = async_backend != "none"
 
-when faststreams_async_backend == "chronos":
+when async_backend == "none":
+  discard
+elif async_backend == "chronos":
   import
     chronos
 
@@ -18,10 +29,10 @@ when faststreams_async_backend == "chronos":
   template fsAwait*(f: Future): untyped =
     await f
 
-elif faststreams_async_backend in ["std", "asyncdispatch"]:
+elif async_backend in ["std", "asyncdispatch"]:
   import
     std/asyncdispatch
-  
+
   export
     asyncdispatch
 
@@ -36,7 +47,7 @@ elif faststreams_async_backend in ["std", "asyncdispatch"]:
   type Duration* = int
 
 else:
-  {.fatal: "Unrecognized network backend: " & faststreams_async_backend.}
+  {.fatal: "Unrecognized network backend: " & async_backend.}
 
 when defined(danger):
   template fsAssert*(x) = discard

faststreams/buffers.nim

@@ -22,8 +22,9 @@ type
     maxBufferedBytes*: Natural
 
     queue*: Deque[PageRef]
-    waitingReader*: Future[void]
-    waitingWriter*: Future[void]
+    when fsAsyncSupport:
+      waitingReader*: Future[void]
+      waitingWriter*: Future[void]
 
     eofReached*: bool
     fauxEofPos*: Natural