[feature] crypto library implementation compatible with exist 7

[line-o]

TBD

[adamretter]

Out of interest, where are you seeing incompatibilities? Possibly the Type and Cardinality classes?

[adamretter]

@line-o We have already done this for Elemental. First step is to make a release compatible with eXist-db 6.x.x, here is a PR for this - eXist-db/expath-crypto-module#87
Once that is merged and released, we can send a follow-up PR by back-porting code to make it work with eXist-db but 7.x.x.