[saffron] Add Diff type , methods, and tests

[martyall]

This PR builds on #3005

While working on #3004 I did not approach the Diff type carefully enough. This PR is meant to slow things down and make sure we have the spec fulfilled before implementing the update functionality that uses it.

• clearly define the Diff type and it's create method using sparse representation suggested by @dannywillems.
• clearly define Diff spec. The assumptions are:
  1. You are allowed to construct a diff if the number of polynomials (chunks) required is less than or equal to the original: test_allow_legal_updates
  2. You are not allowed to construct a diff if the new number of chunks is strictly more -- in this case we should throw an error: test_cannot_construct_bad_diff
• add property based tests to prove the Diff spec

The storage provider will also have to enforce these invariants

[codecov]

Codecov Report

Attention: Patch coverage is 73.10345% with 39 lines in your changes missing coverage. Please review.

Project coverage is 76.83%. Comparing base (505b3b7) to head (82bbc48).
Report is 7 commits behind head on master.

Files with missing lines	Patch %	Lines
saffron/src/diff.rs	70.67%	39 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3006      +/-   ##
==========================================
+ Coverage   76.80%   76.83%   +0.03%     
==========================================
  Files         261      262       +1     
  Lines       61888    62028     +140     
==========================================
+ Hits        47530    47662     +132     
- Misses      14358    14366       +8     

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[martyall]

@dannywillems 82bbc48 was added and PR description updated since original review