blockstore: Use Vec::from() instead of slice::to_vec()

[steviez]

Problem

Vec::from() on a Box<[T]> takes over the heap allocation whereas slice::to_vec() copies the contents into a new Vec. We don't need the old Box<[u8]> so transferring ownership is sufficient here

Summary of Changes

Use Vec::from() instead of to_vec(). Playing with it on godbolt, we save a movzx when optimization cranked up. So, not something that will move the needle on TPS or anything, but I view this more as a correctness thing too.

https://godbolt.org/z/Wbs6v6vEs