Add store providers to persistence module

[dr-orlovsky]

This solves the problem with saving and restoring Stock and related persistence objects in a custom way.

If a client of the library want to use custom store and load procedures, it has to do the following:

• create a custom type with all necessary information;
• implement StoreProvider for with MemStash, MemState and MemIndex generics
• implement StockStoreProvider for it

You can check how it is done in https://github.com/RGB-WG/rgb-std/compare/store?expand=1#diff-ad8cec98dd4a1dac3d2a20e6c7a9c3e64129a3182285b50e1e3df20da2c167c7 where PathBuf becomes such store provider for a local file. If somebody needs to use Amazon S3 for instance, they can wrap this PathBuf into a custom structure and use its methods when needed.

Two other related PRs:

• Abstract wallet persistence with external persistence providers BP-WG/bp-wallet#66
• Use new StoreProvider APIs rgb#238

[will-bitlight]

Dear Maxim, I have a colleague who is paying attention to this PR at any time and cooperating with the docking work of the client. You can replace him @Crayon-Shin-chan-1024 as reviewer, so that your design can have a better understanding with the downstream projects :)

[dr-orlovsky]

Thank you for the suggestion! First, he needs to accept invitation to the org

[codecov]

Codecov Report

Attention: Patch coverage is 4.81928% with 158 lines in your changes missing coverage. Please review.

Project coverage is 19.2%. Comparing base (91a3c41) to head (47e81b9).
Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
src/persistence/memory.rs	13.1%	53 Missing ⚠️
src/persistence/stock.rs	0.0%	48 Missing ⚠️
src/persistence/fs.rs	0.0%	42 Missing ⚠️
src/persistence/index.rs	0.0%	5 Missing ⚠️
src/persistence/stash.rs	0.0%	5 Missing ⚠️
src/persistence/state.rs	0.0%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #251     +/-   ##
========================================
- Coverage    19.4%   19.2%   -0.2%     
========================================
  Files          39      40      +1     
  Lines        8610    8604      -6     
========================================
- Hits         1672    1655     -17     
- Misses       6938    6949     +11     

Flag	Coverage Δ
rust	19.2% <4.8%> (-0.2%)	⬇️

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