Change checkers to become objects similar to decode

[bee-san]

Problem

When we're using Caesar cipher, with the old design we'd need to do something like this:

def caesar():
    output = []
    for i in range(0, 26):
        output + crack()
    return output
    
check(output)

This is a bad idea because:

1. We should align the happy path to the left and exit as soon as we find a solution.
2. Decoders do not support returning multiple items so we would need to edit them to do so.

I propose a different solution:

def caesar(text, checker):
    output = []
    for i in range(0, 26):
        cracked_item = crack()
        if check(cracked_item):
            return success
    return failure

This saves time, is cleaner and gives decoder authors ultimate control over when we check if its plaintext or not.

This pull request turns checkers into objects so we can pass them to the decoders. We do not check in the decoders yet, that is for another PR.