crypto.rc4: new_cipher should return a reference to the Cipher for consistency with other ciphers

[einar-hjortdal]

if x.crypto.chacha20 new_cipher returns !&Cipher, then all new_cipher functions that return a cipher.Stream implementation should return a reference for consistency.

From here

struct WireChannel {
mut:
	conn          &net.TcpConn
	reader        &io.BufferedReader
	writer        &io.BufferedWriter
	plugin        string
	crypto_reader &cipher.Stream
	crypto_writer &cipher.Stream
}

fn (mut c WireChannel) set_crypt_key(plugin string, session_key []u8, nonce []u8) ! {
	c.plugin = plugin
	match plugin {
		chacha20_64_plugin_name, chacha20_32_plugin_name {
			mut digest := sha256.new()
			digest.write(session_key)!
			key := digest.sum([]u8{})
			c.crypto_reader = chacha20.new_cipher(key, nonce)!
			c.crypto_writer = chacha20.new_cipher(key, nonce)!
		}
		rc4_plugin_name {
			// unlike assignment with chacha20.new_cipher, here the user needs 2 steps to assign a reference
			r := rc4.new_cipher(session_key)!
			w := rc4.new_cipher(session_key)!
			c.crypto_reader = &r
			c.crypto_writer = &w
		}
		else {
			return error(format_error_message('Unknown wire encryption plugin name: ${plugin}'))
		}
	}
}

With my proposed change, code looks cleaner and more consistent

fn (mut c WireChannel) set_crypt_key(plugin string, session_key []u8, nonce []u8) ! {
	c.plugin = plugin
	match plugin {
		chacha20_64_plugin_name, chacha20_32_plugin_name {
			mut digest := sha256.new()
			digest.write(session_key)!
			key := digest.sum([]u8{})
			c.crypto_reader = chacha20.new_cipher(key, nonce)!
			c.crypto_writer = chacha20.new_cipher(key, nonce)!
		}
		rc4_plugin_name {
			c.crypto_reader =  rc4.new_cipher(session_key)!
			c.crypto_writer =  rc4.new_cipher(session_key)!
		}
		else {
			return error(format_error_message('Unknown wire encryption plugin name: ${plugin}'))
		}
	}
}

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