Rebasing and adding initial commit of the Concurrency Chapter

Author: borsakv

src/concurrency.md

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+r[concurrency]
+# Concurrency
+
+r[concurrency.intro]
+Rust provides language and library features for writing concurrent programs. These features are designed to prevent data races --- situations in which multiple threads access the same memory without proper synchronization, with at least one of the accesses modifying that memory.
+
+This chapter describes the traits, types, and concepts that Rust uses to express and enforce safe concurrency.