fix: range operator precedence and nested range with ternary step

[doorgan]

Fixes one of the errors found by #78

Spitfire.parse("a..b..c//d") is producing a..(b..c//d) instead of (a..(b..c))//d.

This is due to the wrong precedence being used for @range_op, and the parsing of nested ranges causing the inner range to consume the // operator.