Fix a typo in an unsafe impl Send/Sync

[alexcrichton]

This commit fixes a mistake in the PrePatchedFuncRef type where it has an unsafe impl to make it send/sync but the target of the impl was mistakenly InstancePre<T>

Note that this doesn't actually have any impact on the send/sync-ness of InstancePre<T> since it's not storing an instance of T, so it's always Send/Sync anyway. Nevertheless this reduces the scope of unsafety slightly as was originally intended.

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[jameysharp]

This makes sense. But since this was passing CI before, I guess that nothing was depending on PrePatchedFuncRef being Send or Sync, so do we need those at all? Could these unsafe impls just be deleted instead?

[alexcrichton]

Oh it was InstancePre<T> that depends on that impl. The previous impl though was unsafe impl<T> Send for InstancePre<T>, which led me to believe that these impls were still necessary.

That being said if I remove the impls InstancePre<T> is indeed Send/Sync as expected. My guess is that while this was historically necessary refactorings ended up making it not necessary!

I'll update this to remove the wrapper entirely