Why `foldl` is not working with `openarray`, seems like it should work.

[shirleyquirk]

in the case of `expensiveProc().foldl(a+b)` foldl will not evaluate 
`expensiveProc()` `len` times. Instead, it assigns the sequence to a local 
variable and performs the operation on that copy but an openArray is not a 
firstclass type (yet?) and can't be copied.

many ways to make this work: 
    
    
    proc sum*(values: openArray[float]):float = values.toSeq.foldl(a+b)
    
    template sum*(values: openArray[float]):float = values.foldl(a+b)
    
    template sum*(values: untyped): untyped = values.foldl(a+b)
    
    
    Run