[V2.0] HDF Update and TopicHash Introduction

[DyrellC]

Description of change

Addresses #260 and #256. Adds a MAC to the HDF for additional verification as well as adding the new TopicHash struct. CursorStore now maps branches by TopicHash instead of Topic, and HDF's now contain the Topic as TopicHash format instead of plaintext.

Type of change

• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How the change has been tested

• Previous examples continue to work
• Tests all continue to pass
• fmt/clippy

[DyrellC]

Originally the plan was to include #259 in this PR as well, but what I have found is that there is no reasonable automated scenario that a user that would need to read the Keyload would be able to sequence to and see that Keyload without having known the Topic of the branch first, thus defeating the purpose of carrying the Topic through the Keyload @arnauorriols

[DyrellC]

As a consequence of the above stated, it brings into question the need to store cursors by TopicHash at all, as the reason for that was to accomodate behavior brought forward by #259.

[kwek20]

Small change and then good to go!

[kwek20]

Looking at the code, this didnt need to be cloned really. a reference is enough

[DyrellC]

If we use a reference we need to clone each time anyways to set the latest link for the branch each time a message is handled