Implicit returns generates warnings
```nim
import chronos
proc testFoo1(): Future[int] {.async.} =
  result = 3
proc testFoo2(): Future[int] {.async.} =
  return 3
proc testFoo3(): Future[int] {.async.} =
  3
when isMainModule:
  echo waitFor(testFoo3())
```
This program generates compilation warning
```
nim-chronos\chronos\asyncmacro2.nim(180, 44) Hint: 'result' is declared but not used [XDeclaredButNotUsed]
```
`testFoo1()` and `testFoo2()` do not generate warnings.

Related PR https://github.com/status-im/nim-chronos/pull/401

cc @arnetheduck 