Nim implementation of Ruby tap() method

[demotomohiro]

I don't know about Ruby but `tap()` looks like [Statement list 
expression](https://nim-lang.org/docs/manual.html#statements-and-expressions-statement-list-expression)
 or [Block 
expression](https://nim-lang.org/docs/manual.html#statements-and-expressions-block-expression).
 But they cannot be chained like proc call.
    
    
    var num = 123
    echo (echo num * 2; num)
    echo (echo (echo num * 3; num) * 4; num)
    echo block:
           echo num * 4
           num
    
    Run