Interface definition for threading

The threading interface header requires the following:

```c
struct _NpmThread {
    void *id;
    int Reap(void);
    int Prune(void);
};

typedef _NpmThread NpmThread;

NpmThread* Sow(void (*entry)(void *),
               void *args,
               uint32_t argLen);
void Cleanup(NpmThread** th);
```

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- [ ] GTest for the interface