[WIP] Initial implementation of blocktree

[creativcoder]

Summary of changes

The blocktree is a data structure used to store the candidate chains when performing chain selection during syncing of a mina node. Equivalent to a transition frontier in mina ocaml impl.

Changes introduced in this pull request:

• Adds a base port of gosammer's blocktree implementation.
• About 60% of test cases pass.

Todo:

• Fix remaining test cases
• Remove unwraps
• Implement the Blocktree trait
• Identify how to adapt it to mina's use case.

Reference issue to close (if applicable)

Closes: #30

Other information and links