[saffron] Add Read Proof

[martyall]

• Add functionality for creating / verifying read proofs according to @marcbeunardeau88 's diagram. NOTE: This only works for slice reads right now (think of the form my_data[start..end]), not a random subset of indices. I discussed this with @marcbeunardeau88 as being the easiest thing to do that was useful.
• Add prop tests to check on random data with random queries
• Add a proof module namespace, i.e.

saffron/src/proof
├── mod.rs
├── read.rs
└── storage.rs

TODO

This currently creates a "trivial" read proof for a given chunk if that chunk doesn't contain any of the query indices (effectively 0 == 0) . There is surely a smarter / faster way to do this using Option that is also sound, proposing to do in a follow up PR.

[codecov]

Codecov Report

Attention: Patch coverage is 96.96970% with 5 lines in your changes missing coverage. Please review.

Project coverage is 77.07%. Comparing base (7da4dc0) to head (304b505).

Files with missing lines	Patch %	Lines
saffron/src/main.rs	0.00%	3 Missing ⚠️
saffron/src/proof/read.rs	98.66%	2 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           martin/saffron-separate-query-module    #3016      +/-   ##
========================================================================
+ Coverage                                 76.84%   77.07%   +0.23%     
========================================================================
  Files                                       263      264       +1     
  Lines                                     62191    62348     +157     
========================================================================
+ Hits                                      47791    48056     +265     
+ Misses                                    14400    14292     -108     

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