Add powdr- prefix to crate names

[leonardoalt]

I want to start preparing the crates for release and publishing on crates.io . The first convention is to name all crates powdr-*.

For now I only renamed the crates in Cargo.toml files, so the code doesn't compile. If we decide that this is fine and decide on some conventions, I'll go ahead and change the code as well.

[pacheco]

I don't have a preference either way, but it's also possible to rename the package used for the crate in the .toml file, to avoid using powdr_* in the code:
ast = { package = "powdr_ast", path = "../ast" }

[leonardoalt]

I think I'd prefer to use the powdr- prefix everywhere to make it obvious what's from powdr