In the DSP/algorithm domain, a time-stretching system—both real-time and offline—would be extremely useful.
I agree, i personally think that time stretching and pitch shifting are complex enough that its worth making your own version (even if its bad) to just understand how it works.
There are also 3rd party libraries you can use for things like this. I believe that SignalSmith has one on github actually: GitHub - Signalsmith-Audio/signalsmith-stretch: C++ polyphonic pitch/time library (GitHub mirror)
this is a mirror from another source control but its still the same code and should function the same!
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There are a lot of things that would be useful to have the source code for, but stuff that’s really good and special is usually kept a closely guarded secret by its creators because it’s worth a lot of money. 
signalsmith i think did the best free implementation you can watch his video for ADC he has link to his code as well. Signalsmith Stretch : Open-source code : Signalsmith Audio. I am in the process of trying to replace it with a time domain version as I like the artifacts better.
Edit: direktdsp already posted it.
Yes, I know that library — it’s fantastic. My proposal for JUCE is to have new tools available out of the box.
Yes, that’s understandable. JUCE already offers a lot to us developers. I proposed this idea because sometimes integrating third-party libraries takes more time than the entire project itself. Having ready-to-use tools would be a great help.
My proposal for JUCE is to have new tools available out of the box.
In contrast I would prefer it if juce gave us NO DSP functions at all. 
( Other than the SIMD stuff. )
Sorry,
Dave H.