Add GetFirstBlock and GetLastBlock to IBlock

[Washi1337]

Summary:
There is currently no easy way to get the first and last block of an IBlock instance. The only way is to either pattern match, or use GetAllBlocks() and get the first or the last block from there. This can be very inefficient with large scope blocks.

The proposal is to add two methods GetFirstBlock and GetLastBlock to the IBlock interface.