Add raiseMUnderN to freer-extras

[sjoerdvisscher]

This may seem silly, but I needed this for M = 6 and N = 3.

The mapEffs function seems generally useful, so I suggested to add it to freer-simple: lexi-lambda/freer-simple#37

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[michaelpj]

I don't remember any of this, but I trust that you know what you're doing, and if it compiles it's probably fine.

[sjoerdvisscher]

@michaelpj Lol, your memory has a healthy bit of garbage collection then!