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

tests/test_outputs.nim

@@ -154,6 +154,15 @@ suite "output stream":
 
     checkOutputsMatch()
 
+  test "memcpy":
+    var x = 0x42'u8
+
+    nimSeq.add x
+    memStream.writeMemCopy x
+    let memStreamRes = memStream.getOutput
+
+    check memStreamRes == nimSeq
+
   template undelayedOutput(content: seq[byte]) {.dirty.} =
     nimSeq.add content
     streamWritingToExistingBuffer.write content