Fix `unstable` feature.

[zachs18]

Currently, enabling the unstable cargo feature of thin-vec does not enable the Rust nightly feature trusted_len, so cargo +nightly build --features=unstable always fails. This PR adds #![cfg_attr(feature = "unstable", feature(trusted_len))] to remedy that.

Alternately, the fact that this hasn't been reported may indicate that the unstable feature is not used in the wild and could just be removed (or leave the feature itself and only remove the two TrustedLen impls gated under it).