Add GCM

[AKushWarrior]

In steel_crypt, I pulled GCM from a fork of PointyCastle.

Here's the associated PR: PointyCastle/pointycastle#153

[AKushWarrior]

I'll rebase + touch up this PR and notify when done.

[AKushWarrior]

@mwcw this is done and ready for merging in the linked PR.

[mwcw]

Thanks..

I have merged the PR.

MW