use for loop instead of while in SEQUENCE 

This avoids side effects:

   my @abc = ^10; my @ = @abc ... 4; say @abc;

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using binding for better performance

There were concerns about loss of performance with for loop instead of while/shift.  A binding instead of an affectation might help.
Merge pull request #273 from grondilu/patch-2

use for loop instead of while in SEQUENCE