Refactored grains to use bigint

[joshiraez]

Fixes #21

• Modified sample solution to use bigint
• Modified tests to expect for big int
• Added a note on the instructions to use bigint with a link to the docs.

I'm unsure what else would be needed or how to run the tests - the yarn command in README.md I'm not sure what it does, but it fails 😅 . If anyone can tell me if there is something else missing or how to run the test, I can check that out/finish it. Thanks!

[SleeplessByte]

@joshiraez ah yes. I will fix all of those this weekend and ping you (here and on Slack) and then ALL SHOULD WORK :D :D 🗡️

[SleeplessByte]

@joshiraez I've rebased the PR, fixed the proof, and added you to the contributors list for this exercise. For future reference:

1. Go into the root of the repository and yarn install
2. From the root of the repository, you can now run all the scripts:

# Testing a single exercise
ASSIGNMENT=practice/grains yarn babel-node scripts/test

Running tests for practice/grains...

PASS tmp_exercises/grains.test.ts
  returns the number of grains on the square
    √ 1 (2 ms)
    √ 2
    √ 3
    √ 4
    √ 16
    √ 32 (1 ms)
    √ 64
    √ square 0 raises an exception (3 ms)
    √ negative square raises an exception
    √ square greater than 64 raises an exception (1 ms)
  returns the total number of grains on the board
    √ total

# Testing all exercises
 yarn babel-node scripts/test

If you incorrectly fill in a slug (like in the CI right now), it will guess:

$ ASSIGNMENT=grains yarn babel-node scripts/test

[Failure] that's not a valid assignment reference
Expected {type}\{slug}, actual: grains.
- Use concept\grains if grains is a concept exercise.
- Use practice\grains if grains is a practice exercise.

Did you mean:
- practice\grains