Feature/33/derive enum variants

[azriel91]

Implemented #33.
there's an error that I can't work out, see commit 429040a, uncommenting that code causes the compiler to panic (it doesn't find the strum attribute, even though it's clearly defined). Fixed it.

This branch was branched off #32, so it's a bit noisy from that.

[azriel91]

Solved the issue I hit earlier and added docs, phew. Also allow the user to rename the generated enum.

[azriel91]

okay, finally got this working:

#[strum_discriminants(
    // These attributes are specifically looked for
    name(MyVariants),
    derive(EnumIter, ToString),
    // --- Any additional attributes are passed through to the derived enum
    strum(serialize = "snake_case"),
)]
enum MyEnum { A }

// generates
#[derive(Clone, Copy, Debug, PartialEq, Eq, EnumIter, ToString)]
#[strum(serialize = "snake_case")]
enum MyVariants { A }

[Peternator7]

Changes available on crates.io :) Thanks for all your help on this!!