Samll rework of BlockData type class

[Shimuuar]

• Lenses for Block are added
• Newtype for time difference is added
• BlockException is type family now
• Block construtor is rename to Block
• thresholdRetarget method dropped
• runNode is not used anymore and dropped

Block mining needs further rework. For example block creation function may need extra parameters like coinbase transaction etc. trying to encode them in BlcokData type class is pain so it's left as future work