RC4 implementation for Crystal

Adapted from ruby/rc4 Usage (after adding to shard.yml):

require "rc4"

key = "your key".to_slice
data = "some data".to_slice

# to encrypt
rc4_enc = RC4.new key
encrypted = rc4_enc.stream data # Slice(UInt8)

# to decrypt
rc4_dec = RC4.new key
decrypted = rc4_dec.stream data

encrypted == decrypted # true