Add "collections" exercises

[sazid]

Since Vec and HashMap are two fundamental data structures that are used often in every programming languages, I added 4 new exercises for them.

• The "collections" exercises has been added after the "modules" and before the "macros" exercises.
• vec1.rs and vec2.rs for exercises related to Vectors. First exercise focuses on declaration of vectors and second exercise focuses on iteration and mutability.
• hashmap1.rs and hashmap2.rs exercises for HashMaps. First exercise focuses on declaration and insertion. Second exercise focuses on entry() API.

Open to any improvements or suggestions!

[AbdouSeck]

Hi @sazid, thanks for these exercises. They're very useful. I have two suggestions:

1. Would you consider squashing your commits so that you end up with 2 commits: 1 commit for the HashMap exercises and 1 commit for the Vec exercises?

2. I see that you're converting the keys of hash maps into vectors just to get the length/size of the hash maps, but the HashMap type already offers .len() which can take care of that. For instance, this line of code could have been shortened with assert!(basket.len() >= 3). Also, I have noticed other ways of shortening lines of code. I made additional comments above.

Other than these two things, there is really nothing else I can comment on.

Thanks,
Abdou

[sazid]

Hi @sazid, thanks for these exercises. They're very useful. I have two suggestions:

1. Would you consider squashing your commits so that you end up with 2 commits: 1 commit for the HashMap exercises and 1 commit for the Vec exercises?
2. I see that you're converting the keys of hash maps into vectors just to get the length/size of the hash maps, but the HashMap type already offers .len() which can take care of that. For instance, this line of code could have been shortened with assert!(basket.len() >= 3). Also, I have noticed other ways of shortening lines of code. I made additional comments above.

Other than these two things, there is really nothing else I can comment on.

Thanks,
Abdou

1. Will do.
2. I completely forgot about the HashMap's len() method! I'll adjust them as well and push changes.

Thanks for your suggestions!

[sazid]

@AbdouSeck I've resolved the issues and also squashed into two separate commits.

Thanks.

[AbdouSeck]

@sazid looks good to me. Thank you!

@fmoko I think this can be merged.

Abdou

[shadows-withal]

Thanks for the exercises, and @AbdouSeck for the review!

[shadows-withal]

@all-contributors please add @sazid for content and code

[allcontributors]

@fmoko

I've put up a pull request to add @sazid! 🎉