If we use `#![cfg_attr(docsrs, feature(doc_auto_cfg))]` instead of
`#![cfg_attr(docsrs, feature(doc_cfg))]` we no longer need to manually
mark types with `#[cfg_attr(docsrs, doc(cfg(feature = "std")))]`.
Sweeeeeet.
We recently created a `types` subdirectory under `script` to keep all
the `Script` and `ScriptBuf` impls together. Turns out this additional
level of subdirectory is a bit annoying and we can achieve the same
grouping by just using `script/mod.rs`.
Move code from `types/mod.rs` to `script/mod.rs`, move the two
submodules up a level, remove the `types` directory.