Fix some implicit int64 to int conversion warnings. However, the real issue
is that PermutationMatrix mixes the type of the stored indices and the "Index"
type used for the sizes, coeff indices, etc., which should be DenseIndex.
(transplanted from eae4abe46e5b454adfad59f9894f340594c1b9b7)

--HG--
extra : transplant_source : %EA%E4%AB%E4n%5BEJ%DF%ADY%F9%89O4%05%94%C1%B9%B7