move to cargo workspace + small readme edits

[matt-codecov]

getsentry/ophio uses a Cargo workspace to separate the core library from the PyO3 bindings. following suit here. benches, tests, and examples all can be invoked same as before, no code needed to change

also removed mentions of mockall from the readme and made other small edits

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• initialize maturin package #28
• -> move to cargo workspace + small readme edits #27

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (87b8459) to head (595a8df).
Report is 2 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files          17       17           
  Lines        7315     7315           
=======================================
  Hits         7219     7219           
  Misses         96       96           

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