replace partial with an anonymous fn ((small) performance optimisation)

Hi Peter, 

It's a small detail but it seems partial induces a little overhead compared to an anonymous fns.

Some numbers for `roundrip`

partial : 
13362
13346
13452

ano fn: 
10988
10897
10951

The gain is not incredible, but it's there. Even tho I also find partial more elegant, it has a small cost and hotspot seems not to want to optimise that.