Reorder files generated by “moments_varendo” option of “estimation”

If there were more than 10 files of Metropolis parameter draws, the ordering
the files containing the posterior moments could be different from that of the
parameter draws. This is because the “dir()” command was used to order the
files containing the parameter draws, and because the command uses alphabetic
ordering, file #10 would come before #2.

This commit enforces the numerical ordering of files.