Confusing error message when calling toSeq using a non-iterator

```nim
import sequtils

type A = object
iterator items(self: A): int = discard
discard toSeq(A())  #type mismatch: got (seq[A], int)
```
This works:
```nim
discard toSeq(A().items)
```
Normally "toSeq" calls the "items" iterator if it finds one on it's own.
```nim
toSeq(1..10)
```
