Convert library to doctrine/orm 3

[Firehed]

Like doctrine/orm itself, this does include a major BC break: the default driver has moved from annotations to attributes, as the former has been removed.

Most of this is adapting to the upstream interface changes and changing some documentation. The core of the internal logic (query resolving, etc) is largely unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 35.29412% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.03%. Comparing base (86f0f10) to head (9e9ae86).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/InMemoryEntityManager.php	13.15%	33 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #43      +/-   ##
============================================
+ Coverage     67.37%   70.03%   +2.65%     
+ Complexity      134      131       -3     
============================================
  Files             5        6       +1     
  Lines           328      317      -11     
============================================
+ Hits            221      222       +1     
+ Misses          107       95      -12     

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