Merge ES6 branch - #265

* Revert "Replace var with const and let in accessing-array-values (#228)"

This reverts commit cf34563.

* Revert "switched number-to-string to const (#230)"

This reverts commit 65d5e7b.

…pectively (#234)

* Update 'string' problem to use 'const'

* Update 'for-loop' problem to use 'let'

* Changed 'var' to 'let' in for-loops

* fix: 'var' to 'let' & 'const' in solutions

Replace ‘var’ with const.

Replace ‘var’ with ‘const’

changes var to let in variables

Updates var to const in the strings exercise set

update variable declaration for number-to-string problem