Closes #44: Abstract spongy castle

[rmeissner]

Implements walleth/kethereum#44

• Created a crypto_api module that defines interfaces for all underlying crypto functions
• Created a crypto_api_spongycastle module that uses SpongyCastle to implement the interfaces defined in the crypto_api module
• Create a crypto_api_wrapper module that provides an implementation for each abstraction defined in the crypto_api. This allows us to switch over gradually and at some point the wrapper could be removed completely.
• Add interfaces for javax.crypto.* classes to crypto_api
• Created crypto_api_java_provider to use javax.crypto providers to implement some of the interfaces

[rmeissner]

I used objects since most of the implementations don't need a state and therefore I avoid that they need to be recreated each time you access them.

Any specific reason why you prefer classes (I am just curious 😄 )

[ligi]

:+1: thanks - great work!

I prefer classes to objects as I hate global state - and objects tend to introduce global state