Update R6RS package and create R7RS package

@weinholt The code seems ready for release. Can you update the Akku R6RS package and test that it doesn't break the spdx package?

I can create an R7RS package and submit it to Snow-Fort.

@mario-goulart Can update the Chicken egg.

Since Akku pulls packages from Snow-Fort, how should we handle the conflict between R6RS and R7RS?