Update problems to use const and let

[shisama]

resolve #660

'var' is used in some problem.md files. I think 'let' or 'const' should be used instead of 'var'.

[shisama]

Is there anybody who can review this?

[shisama]

@AnshulMalik Hi, I created this PR 5 months ago. Please review this PR and merge it when you have time. Thanks.

[AnshulMalik]

Hi @shisama, sorry it went unnoticed for so long. Probably because of so many file changes together. I am so sorry for not having a look at it.
I think the exercises are already converted by community.

[shisama]

@AnshulMalik Thank you for your reply.

Probably because of so many file changes together.

Is it better for you to divide this PR each exercise? For example, one is PR to update exercises/http_collect, two is exercises/http_file_server, ...

I think the exercises are already converted by community.

OK, I will change the latest code again.

[AnshulMalik]

Yes @shisama one PR per exercise is generally best. Moreover not many exercises are left now :)