Implement BLS aggregate signature verification

[austinabell]

Individual signature verification was done in #40 but the aggregate signature verification needs to be implemented when the block's aggregate BLS signature needs to be verified. Should be pretty straightforward to expand logic done in that PR to verify an aggregate but if anyone picks this up before me I can give an example of how to do this.

The reason this isn't being done now is because it is not clear the exact usage or data inputs to be verified. It can be inferred or kept very basic, but since there is no need for this now it will be left as an issue.