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_inputs.nim

@@ -2,15 +2,11 @@
 
 import
   os, unittest2, strutils, random,
-  stew/ranges/ptr_arith, testutils,
+  testutils,
   ../faststreams, ../faststreams/textio
 
 setCurrentDir getAppDir()
 
-proc bytes(s: string): seq[byte] =
-  result = newSeqOfCap[byte](s.len)
-  for c in s: result.add byte(c)
-
 proc str(bytes: openarray[byte]): string =
   result = newStringOfCap(bytes.len)
   for b in items(bytes):
@@ -28,7 +24,7 @@ const
   asciiTableFile = "files" / "ascii_table.txt"
   asciiTableContents = slurp(asciiTableFile)
 
-procSuite "input stream":
+suite "input stream":
   template emptyInputTests(suiteName, setupCode: untyped) =
     suite suiteName & " empty inputs":
       setup setupCode