Ensure blake3 inputs are immutable

[libitx]

Fixes #50

• ensures the blake3 IV is copied, not referenced
• adds tests ensuring inputs are immutable for all the blake family hashing functions

As discussed in #50, I tried to spot if this issue is anywhere else. Looking where toBytes is used doesn't really help as a) it's used all over the place, and b) it doesn't reveal if elsewhere in the code a referenced input is mutated.

Best thing is to add similar tests as I've done for every hash function. But that's a job for a rainy day ;)

[paulmillr]

please run npm run format

[libitx]

formatting is done 👍