Implement 'scls validate'

[nc6]

Create a new SCLS binary.

The first tool should be 'scls validate' with the following options:

scls validate should validate the whole file, checking that the internally stored digests are correct and then reporting the root of the Merkle tree.

scls validate --namespace should take a list of namespaces to validate

Standard options like --quiet, --verbose should also be added