Signed-off-by: p4u <pau@dabax.net>

- include the db package from aergo to this repository
- move sparse merkle tree code to smt directory
- add a new root package for being compatible with vocdoni/censusTree

Signed-off-by: p4u <pau@dabax.net>

- Code copied from https://github.com/aergoio/aergo
- Make it a go module
- Added Walk and GetWithRoot new methods
- Create custom hasher based on blake256

Signed-off-by: p4u <pau@dabax.net>