Add custom context switch library.

The rationale to do an custom implementation is that the existing solutions are quite a bit slower and/or require more memory.
4 jobs from coninuation in 36 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3464
build_cmake

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  Test
failed #3465
run_tests

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  Sanitizer
skipped #3467
run_address_sanitizer
skipped #3466
run_thread_sanitizer
 
Test run_tests
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