Make the ValueScript source code more rusty

[voltrevo]

I've been learning rust as I've been building ValueScript. I'm getting better, but I'm sure the code would still benefit by being rustier in many places.

At minimum, cargo clippy is generating 470 warnings as I write this (after --fix).

I'm not necessarily sold on doing everything clippy says, but it's a decent guide.

If you want to change something a bit more complex than what clippy is able to comment on, you might want to check with me beforehand. In general, this is supposed to be a good-first-issue so the depth of the change should be limited.