Use multiple processors

[AshleyYakeley]

Just need to call setNumCapabilities I think.

[AshleyYakeley]

Switching on SMP actually makes interpretation about 50% slower. So leaving this switched off.

[AshleyYakeley]

May make this a Pinafore library command. In the mean time you can switch on SMP by passing +RTS -N -RTS to pinafore (probably).