`n => n` causes `fz` to hang, might be good to produce a warning...

[fridis]

Try this

> ./build/bin/fz -e 'n => n' 

does not finish... This is not a bug, type inference determines the result of of n to be void and tail call optimization turns this into an endless loop. Not sure if we can find a good criterion —like contains no effectful operations— to decide if an endless recursion (or an endless loop) is worth a warning.