Breaking changes:
- Rename crate: futures_kind → future_form
- Rename macro: #[kinds] → #[future_form]
- Rename trait: IntoFutureKind → FromFuture
- Rename method: into_kind → from_future

New features:
- Custom FutureForm types supported in macro
- #![no_std] with extern crate alloc

Improvements:
- Remove proc-macro-warning dependency
- Add compile-fail tests (trybuild)
- Add HACKING.md contributor guide
- Add extensions doc module for docs.rs

Proc macro generates Sendable and Local trait impls from a single
generic definition. Supports conditional bounds per variant.
IntoFutureKind abstracts over boxing for extensibility.