chacha20: reexport the `Nonce` type

[kennytm]

In v0.10.0, we see that XNonce and LegacyNonce are exported, but the standard Nonce type remains private to crate.

The Nonce type was available in 0.9.1. https://docs.rs/chacha20/0.9.1/chacha20/type.Nonce.html

stream-ciphers/chacha20/src/lib.rs

Lines 21 to 32 in 637e168

	#[cfg(feature = "cipher")]
	pub use chacha::{ChaCha8, ChaCha12, ChaCha20, Key, KeyIvInit};
	#[cfg(feature = "cipher")]
	pub use cipher;
	#[cfg(feature = "legacy")]
	pub use legacy::{ChaCha20Legacy, LegacyNonce};
	#[cfg(feature = "rng")]
	pub use rand_core;
	#[cfg(feature = "rng")]
	pub use rng::{ChaCha8Rng, ChaCha12Rng, ChaCha20Rng, Seed, SerializedRngState};
	#[cfg(feature = "xchacha")]
	pub use xchacha::{XChaCha8, XChaCha12, XChaCha20, XNonce, hchacha};

The Nonce type is internally accessible from crate::chacha::Nonce.

stream-ciphers/chacha20/src/chacha.rs

Lines 12 to 13 in 637e168

	/// Nonce type used by ChaCha variants.
	pub type Nonce = Array<u8, U12>;