Switch to new Merkle tree implementation

MERGE REQUEST

Overview

Switch the crypto package to using the new library merkletree-blake. This new library is almost the same as the old library, except that some of the interfaces are now hardcoded types. The result is massively fewer allocations when uploading and downloading, which all but eliminates the garbage collection pressure, and results in better overall performance for siad.

Checklist

Review and complete the checklist to ensure that the MR is complete before assigned to an approver.

• All new methods, or updating methods have clear docstrings
• Testing added or updated for new methods
• Any new packages are added to Makefile and .gitlab-ci.yml
• API documentation updated for API updates
• Module README.md updated for changes to work flow
• Issue added to Sia-UI repo for new supporting features
• Changelog updated