st: Update to mf_invtokens on SSC

["Jann, Ben" <ben.jann@soz.gess.ethz.ch>]

Thanks to Kit Baum, the mata function -invtokens()-
has been updated on SSC.

The .mo file has been rebuilt to be compatible with 
the new Stata 9 executable.

To install -invtokens()-, type

 ssc install mf_invtokens, replace   

ben

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