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/multisync.nim

@@ -1,35 +1,40 @@
 import
-  stew/shims/macros,
-  async_backend, inputs, outputs
+  async_backend
 
 export
   async_backend
 
-macro fsMultiSync*(body: untyped) =
-  # We will produce an identical copy of the annotated proc,
-  # but taking async parameters and having the async pragma.
-  var
-    asyncProcBody = copy body
-    asyncProcParams = asyncProcBody[3]
+when fsAsyncSupport:
+  import
+    stew/shims/macros,
+    "."/[inputs, outputs]
 
-  asyncProcBody.addPragma(bindSym"async")
+  macro fsMultiSync*(body: untyped) =
+    # We will produce an identical copy of the annotated proc,
+    # but taking async parameters and having the async pragma.
+    var
+      asyncProcBody = copy body
+      asyncProcParams = asyncProcBody[3]
 
-  # The return types becomes Future[T]
-  if asyncProcParams[0].kind == nnkEmpty:
-    asyncProcParams[0] = newTree(nnkBracketExpr, bindSym"Future", ident"void")
-  else:
-    asyncProcParams[0] = newTree(nnkBracketExpr, bindSym"Future", asyncProcParams[0])
+    asyncProcBody.addPragma(bindSym"async")
 
-  # We replace all stream inputs with their async counterparts
-  for i in 1 ..< asyncProcParams.len:
-    let paramsDef = asyncProcParams[i]
-    let typ = paramsDef[^2]
-    if eqIdent(typ, "InputStream"):
-      paramsDef[^2] = bindSym "AsyncInputStream"
-    elif eqIdent(typ, "OutputStream"):
-      paramsDef[^2] = bindSym "AsyncOutputStream"
+    # The return types becomes Future[T]
+    if asyncProcParams[0].kind == nnkEmpty:
+      asyncProcParams[0] = newTree(nnkBracketExpr, bindSym"Future", ident"void")
+    else:
+      asyncProcParams[0] = newTree(nnkBracketExpr, bindSym"Future", asyncProcParams[0])
 
-  result = newStmtList(body, asyncProcBody)
-  when defined(debugSupportAsync):
-    echo result.repr
+    # We replace all stream inputs with their async counterparts
+    for i in 1 ..< asyncProcParams.len:
+      let paramsDef = asyncProcParams[i]
+      let typ = paramsDef[^2]
+      if eqIdent(typ, "InputStream"):
+        paramsDef[^2] = bindSym "AsyncInputStream"
+      elif eqIdent(typ, "OutputStream"):
+        paramsDef[^2] = bindSym "AsyncOutputStream"
 
+    result = newStmtList(body, asyncProcBody)
+    when defined(debugSupportAsync):
+      echo result.repr
+else:
+  macro fsMultiSync*(body: untyped) = body