Change +, -, *, /, //, and % operators' associativity to left-to-right

[6aJIaJIae4HuK]

Before:

• Expressions like 10 - 2 - 4 parsed as 10 - (2 - 4) instead of more convenient (10 - 2) - 4
• Expressions like 200 / 2 / 2 parsed as 100 / (2 / 2) instead of more convenient (200 / 2) / 2

Now:

• Fixed it
• Added corresponding test

[6aJIaJIae4HuK]

@rmorozov, hi, can you take a look plz?

[6aJIaJIae4HuK]

:(

[rmorozov]

@6aJIaJIae4HuK , sorry, have my first baby born and had no time for optional activities=)

I have big review where I rework logics for internal json parser and dump 3rd party code as well - #279

I plan to bring it to master in these Christmas/New Year holidays and restore pull request build as well, then merge your PR and issue new release