* Support building for no_std using nightly Rust compiler

- Adds a new feature `experimental-thread-local` that enables building for no_std targets.
  If this feature is not enabled, the crate is identical as before,
  still allowing for compilation using Rust stable >= 1.38.0.

- The `experimental-thread-local` feature makes use of experimental features `thread_local`
  and `lazy_cell`, thus requiring a nightly Rust compiler.

- Support for `std::sync::RwLock` is dropped in no_std builds.