[ENH] Block delta (#1823)

## Description of changes

*Summarize the changes made by this PR.*
 - Improvements & Bug fixes
	 - None
 - New functionality
- Adds a BlockDelta which is used to mutate a block. It reads the
contents of a block and then can allows changes to be applied directly
to it.
	 - Adds the ability to apply() a block delta
- Add a BlockProvider, which is the interface for loading and managing
blocks.

## Test plan
*How are these changes tested?*
- [x] Basic unit tests `cargo test`

## Documentation Changes
None