WIP: add SparseMerklePath

[Qyriad]

Describe your changes

This implements a Merkle path representation which compacts empty nodes into a bitmask to save memory, currently as as u64 — do we need to worry about cases with more than 64 empty nodes in a path?

I used the representation suggested by #215 (comment), but without the depth field, as to my understanding it can be calculated as the sum of stored nodes and set bits in the bitmask.

I want to confirm this is the desired approach, so I haven't yet implemented most of the helper and conversion methods and traits.

Closes #215.

Checklist before requesting a review

• Repo forked and branch created from next according to naming convention.
• Commit messages and codestyle follow conventions.
• Relevant issues are linked in the PR description.
• Tests added for new functionality.
• Documentation/comments updated according to changes.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud