export transpose! and ctranspose!

This PR proposes to export `transpose!` and `ctranspose!`. When I was modified these functions a while ago ( #6690 ), I decided not to export them because the original `transpose!` was also not exported. However, given that a lot of the mutating methods are exported, it would make sense to export these as well.

If you agree to merge (before or after 0.3), I can also add them to the docs.