Awkward KZGSettings::new() API

[nazar-pc]

I was upgrading from much older version and noticed that KZGSettings::new() is not really idiomatic Rust.

Specifically for FsKZGSettings it ignored one of the arguments, for 3 other arguments it does memory allocation (with .to_vec() and .clone()) instead of taking owned vectors as arguments directly.

On a similar note FsKZGSettings now has precomputation field that is public and can be set to anything, also for some reason it has Arc in it. Is it really supposed to be set publicly to arbitrary value? If not then it should probably not be public. I only write this because previously I was constructing FsKZGSettings directly and now it that I should be using FsKZGSettings::new() instead to fill precomputation correctly.