Make the creation of TxRingEntry and RxRingEntry const

[thalesfragoso]

This allow for people to create RingEntry on a const context, which saves a considerable amount of stack during initialization if having them in a static.

I also left the non const Default implementations for compatibility and also because of https://github.com/rust-lang/rust/issues/49147, which causes the creation of entries a bit cumbersome since Aligned isn't Copy, so users can use default on non const contexts if they want to.

CC @adamgreig

[bors[bot]]

Build succeeded:

• continuous-integration/travis-ci/push