remove chronos dependency

The `chronos` backend currently does not work as it relies on nesting
`waitFor` calls, which is not supported by chronos itself - as such,
this PR disables the backend in testing and prints a recommendation to
users to avoid it until the problem has been resolved.

faststreams used without `async` continues to work as normal, which also
is the most common way to use it - the dependency on chronos in the
nimble file is unattractive from a user perspective since it also leads
to the the downloading of bearssl and other deps.

Hopefully, by the time the chronos backend can be used, we can also
specify conditional dependencies in nimble files.

Author: arnetheduck

faststreams.nimble

@@ -9,7 +9,6 @@ skipDirs      = @["tests"]
 
 requires "nim >= 1.2.0",
          "stew",
-         "chronos",
          "unittest2"
 
 let nimc = getEnv("NIMC", "nim") # Which nim compiler to use
@@ -31,7 +30,8 @@ proc run(args, path: string) =
 
 task test, "Run all tests":
   # TODO asyncdispatch backend is broken / untested
-  for backend in ["-d:asyncBackend=none", "-d:asyncBackend=chronos"]:
+  # TODO chronos backend uses nested waitFor which is not supported
+  for backend in ["-d:asyncBackend=none"]:
     for threads in ["--threads:off", "--threads:on"]:
       for mode in ["-d:debug", "-d:release", "-d:danger"]:
         run backend & " " & threads & " " & mode, "tests/all_tests"

faststreams/async_backend.nim

@@ -20,6 +20,8 @@ const
 when asyncBackend == "none":
   discard
 elif asyncBackend == "chronos":
+  {.warning: "chronos backend uses nested calls to `waitFor` which is not supported by chronos - it is not recommended to use it until this has been resolved".}
+
   import
     chronos
 
@@ -30,6 +32,8 @@ elif asyncBackend == "chronos":
     await f
 
 elif asyncBackend == "asyncdispatch":
+  {.warning: "asyncdispatch backend currently fails tests - it may or may not work as expected".}
+
   import
     std/asyncdispatch