Custom initialization of ChaCha counter

[xpe]

Would you be open to a PR? The ChaCha reference allows for initializing the counter to arbitrary values, but the current API doesn't allow this. I expect this would be a fairly simple change to the following code...

/// Initialize the ChaCha internal state, with a 256-bit key and 64-bit nonce.
pub const fn chacha_init(key: [u8; 32], nonce: [u8; 8]) -> [u32; 16] {
	let mut state = [0u32; 16];
	state[0] = chacha_pack(CHACHA_TAU, 0);
	state[1] = chacha_pack(CHACHA_TAU, 4);
	state[2] = chacha_pack(CHACHA_TAU, 8);
	state[3] = chacha_pack(CHACHA_TAU, 12);

	state[4] = chacha_pack(&key, 0);
	state[5] = chacha_pack(&key, 4);
	state[6] = chacha_pack(&key, 8);
	state[7] = chacha_pack(&key, 12);
	state[8] = chacha_pack(&key, 16);
	state[9] = chacha_pack(&key, 20);
	state[10] = chacha_pack(&key, 24);
	state[11] = chacha_pack(&key, 28);

	// 64-bit counter
	state[12] = 0;
	state[13] = 0;
	// Nonce
	state[14] = chacha_pack(&nonce, 0);
	state[15] = chacha_pack(&nonce, 4);
	state
}

[xpe]

See for example:

main...xpe:nanorand-rs:0.7.0_chacha_counter

[xpe]

@Absolucy hello, would you be open to this change?

I've made an updated pull request for version 0.8 at #51.