Remove short-lived concurrent compatibility module

[antono3]

Summary

• remove the short-lived antono2.memory.concurrent compatibility submodule
• keep synchronized range and buddy allocators exclusively in the root antono2.memory API
• move multithreaded contention and ThreadSanitizer coverage to the root module
• rename the runnable example to match the Synchronized*Allocator types

Validation

• released V 0.5.2 (7647ce1): format verification and vet
• released V 0.5.2: 8/8 test files
• released V 0.5.2: all public API examples
• released V 0.5.2: deterministic benchmark smoke test
• released V 0.5.2 + Clang: AddressSanitizer and UndefinedBehaviorSanitizer, 8/8 test files
• released V 0.5.2 + Clang: ThreadSanitizer root-module contention suite