Bitcoin-explorer

- Fix: special treatment for genesis transaction
- Fix: `get_connected_transaction` and `get_connected_block` now return `Err` if some outpoints can't be found
- Enhancement: add new API `get_block_count`

Fixed py36 compatibility issue.

Critical bug fixes

- improve error handling
- stop iterator correctly
- ensure iterator correct order

Reduce memory usage to 4GB for `iter_block(connected=True)`

Make correction to docstring