Improve rating of difficulty

从 config.json 开始，结合 issue 中提到的练习描述，审查现有的难度值。明确评分是相对的，还是与定义好的学习者水平相关，然后在更新元数据之前，就提议的差异达成共识；完成意味着双方已就量表和修订后的评分达成一致，并且它们已在 config.json 中得到一致反映。

config.json assigns a "difficulty" to each exercise.
Here they are:

1
    hello-world
    leap
    luhn
    space-age
    two-fer
2
    collatz-conjecture
    gigasecond
    grains
    pangram
    raindrops
    reverse-string
    secret-handshake
3
    acronym
    allergies
    armstrong-numbers
    atbash-cipher
    binary
    difference-of-squares
    etl
    grade-school
    hamming
    hexadecimal
    isogram
    matching-brackets
    nucleotide-count
    pascals-triangle
    protein-translation
    rna-transcription
    scrabble-score
    sum-of-multiples
    triangle
    trinary
4
    all-your-base
    binary-search
    robot-simulator
5
    anagram
    bob
    clock
    meetup
    nth-prime
    phone-number
    prime-factors
    queen-attack
    robot-name
    series
    sieve
    word-count
6
    circular-buffer
    complex-numbers
7
    beer-song
    crypto-square
    food-chain
    roman-numerals
10
    binary-search-tree
    say

IMHO some of those ratings are wildly off, e.g.:

• luhn is definitely more difficult than hello-world, leap, or two-fer
• I've mentored space-age several hundred times. Many students struggle with that exercise because they member functions have to be const-qualified and they have to choose a type that can represent the age in seconds for all tests.
• pangram and isogram are very similar, IMHO they are equally difficult.
  (The only thing that makes isogram a little bit tricky is that the description doesn't restrict the characters to ASCII so one has to be careful not to call std::isalpha() or std::tolower() with a char.)
• Are beer-song and food-chain really that complicated? (Frankly, I never understood the appeal of those exercises.)

I'm sure you can easily spot some more discrepancies.

And a more general question: Are the difficulties rated in comparison to each other or do those ratings have some meaning (e.g. 1=for beginners who just wrote their first hello-world, 4=for those who feel comfortable writing classes and functions, 7=complex tasks with multiple requirements)?