`IV` defined twice (not merged) in the library

```
$ nm obj/libblake3.so.0.0 | head -2
200000c8 r IV
20000000 r IV
```

This is because it is defined as `static const uint32_t IV[8] = {…};` in `blake3_impl.h`.

(It’s probably not merged because it has its address taken, or something.)

Ideally, move these into the main `.c` file and give them a namespaced name. (There are more such definitions.)