Fusion isn't reliable

When investigating fusion triggering in my own library I tried the following line:

```
recieve ep2 >-> PBS.map (+1) >-> PBS.map (+1) >-> P.print
```

Using `-ddump-rule-firings` nothing `for`-related ever fired.

On the other hand, this seemed to fire _many_ `for`-related rules:

```
recieve ep2 >-> P.map (BS.append "foo") >-> P.map (BS.append "bar") >-> P.print
```

This fired rules correctly too:

```
recieve ep2 >-> P.map (BS.map succ) >-> P.map (BS.map succ) >-> P.print
```

I don't think `map` from `Pipes.ByteString` is firing rules correctly at all ...