Re-order core exercises

[ErikSchierboom]

This PR is the result of work being done to structure the exercises in a track according to a predefined methodology (this work is not finished). As a result of that work, the core exercises have been reordered a bit. The new ordering is as follows:

core
----
├─ hello-world
│  ├─ hamming
│  ├─ pangram
│  └─ robot-name
│
├─ two-fer
│  ├─ isogram
│  └─ acronym
│
├─ leap
│  ├─ grains
│  └─ perfect-numbers
│
├─ gigasecond
│  ├─ collatz-conjecture
│  ├─ phone-number
│  ├─ scrabble-score
│  └─ meetup
│
├─ series
│  ├─ sum-of-multiples
│  ├─ high-scores
│  ├─ accumulate
│  ├─ difference-of-squares
│  ├─ protein-translation
│  ├─ secret-handshake
│  ├─ largest-series-product
│  ├─ pythagorean-triplet
│  ├─ prime-factors
│  ├─ all-your-base
│  ├─ pascals-triangle
│  ├─ simple-cipher
│  └─ roman-numerals
│
├─ space-age
│  ├─ armstrong-numbers
│  ├─ rational-numbers
│  ├─ triangle
│  ├─ complex-numbers
│  └─ darts
│
├─ raindrops
│  ├─ strain
│  ├─ proverb
│  ├─ beer-song
│  ├─ sieve
│  └─ house
│
├─ bob
│  ├─ error-handling
│  ├─ rotational-cipher
│  ├─ twelve-days
│  ├─ anagram
│  ├─ word-count
│  ├─ isbn-verifier
│  ├─ bracket-push
│  ├─ yacht
│  └─ affine-cipher
│
├─ nucleotide-count
│  ├─ etl
│  ├─ parallel-letter-frequency
│  └─ alphametics
│
├─ allergies
│  ├─ food-chain
│  ├─ tree-building
│  ├─ crypto-square
│  ├─ ledger
│  └─ variable-length-quantity
│
├─ binary-search
│  ├─ change
│  └─ dominoes
│
├─ grade-school
│  ├─ list-ops
│  ├─ flatten-array
│  ├─ binary-search-tree
│  ├─ atbash-cipher
│  └─ sublist
│
├─ clock
│  ├─ queen-attack
│  ├─ robot-simulator
│  ├─ bank-account
│  ├─ simple-linked-list
│  ├─ linked-list
│  ├─ dot-dsl
│  └─ dnd-character
│
├─ kindergarten-garden
│  ├─ matrix
│  ├─ minesweeper
│  ├─ scale-generator
│  ├─ ocr-numbers
│  ├─ spiral-matrix
│  ├─ tournament
│  ├─ poker
│  ├─ rectangles
│  ├─ wordy
│  ├─ connect
│  ├─ say
│  └─ sgf-parsing
│
├─ saddle-points
│  ├─ palindrome-products
│  ├─ hangman
│  ├─ diamond
│  ├─ two-bucket
│  ├─ react
│  └─ go-counting
│
├─ markdown
│  ├─ grep
│  ├─ circular-buffer
│  ├─ luhn
│  ├─ run-length-encoding
│  ├─ word-search
│  ├─ bowling
│  ├─ transpose
│  ├─ zipper
│  ├─ forth
│  ├─ pov
│  └─ rest-api
│
├─ book-store
│  ├─ custom-set
│  ├─ nth-prime
│  ├─ pig-latin
│  ├─ rail-fence-cipher
│  ├─ diffie-hellman
│  └─ zebra-puzzle

bonus
-----
reverse-string
rna-transcription

The most notable changes are:

• two-fer: moved up from position 6 to 2. This was always a wrongly ordered exercise, with its solution being far simpler than some of its preceding core exercises. It has not been moved to position 2, as it is not only simple to solve, but is also a natural extension of the preceding hello-world exercise.
• series: has become a core exercise. Its sole goal is to introduce users to collection's in C#, which we previously did not have a good, simple exercise for.
• gigasecond: has become a core exercise. Its goal is to introduce the DateTime type early on, as it is a very important, core data type in c#.
• space-age: has become a core exercise. Its goal is to introduce floating point numbers relatively early.
• sum-of-multiples: no longer a core exercise. This is due to a recently added policy to exclude "mathy" exercises as core exercise.
• accumulate: no longer a core exercise. This is due to the fact that what it tries to teach (laziness) is a rather advanced concept that requires more advanced knowledge of the language, but the actual solution is so simple that advanced users would solve it without giving it a moment's thought.

The current ordering is not perfect, there is still lots to improve, but I feel it is definitely an improvement over the current situation. I'm interested in hearing your thoughts!