85942c355d Re-export block::Header as BlockHeader (Tobin C. Harding)
Pull request description:
For users who want to just grab stuff from the crate root it makes total sense for there to be a `BlockHeader`.
Another nice thing, in the HMTL docs it makes BlockHeader be in the struct list right along with `Block`, `BlockHash`, and `BlockHeight`.
Close: #3548
ACKs for top commit:
apoelstra:
ACK 85942c355dfdcf9ddf9a46202f4db56794dcc85d; successfully ran local tests
jamillambert:
ACK 85942c355d
Tree-SHA512: 6fbaf7936062323d31940179ffcbc62951b21f53955a42dca2e28476bda0cebeb041db0e1b4f1ceeac32e43d94fe5476ef055595ae88dc6cfc266d32082bdf4d