oxidize: Factor into standard Rust crate structure

- Adds initial `Cargo.toml` (and `Cargo.lock` to `.gitignore`)
- Deletes `tool` (it seems hard to `corrode`)
- Moves `lib/tests` => `tests` (we should figure out how to translate them)
- Moves `lib` => `src` to match Rust conventions
- Renames `lib/ykpiv.rs` => `src/lib.rs` to match Rust conventions
- Adds copyright from `ykpiv.h` to the top of all `*.rs` files

Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "yubikey-piv"
+version = "0.0.0"
+authors = ["Tony Arcieri <bascule@gmail.com>", "Yubico AB"]
+edition = "2018"
+license = "BSD"
+
+[dependencies]