[go-nuts] too many concatstring and gostringsize calls

One big improvement would be, instead of setting seq and qual to nil
on line 55, resetting their lengths. In other words:

seq = seq[:0]
qual = qual[:0]

This way you're reusing space you've already allocated instead of
allocating new space. This cuts about 20% of the processing time for
some sample file I found from a Google search.

I get another measurable improvement using bytes.SplitN(last, space,
1) since you're only using the first result from bytes.Split. You
might be able to do even better with bytes.IndexByte, but I had
trouble getting it to work with my sample file.

As for the closures, I think it would be more idiomatic (and
potentially more efficient) to create objects instead of closures.
Closures and single-method objects are basically equivalent, but when
in doubt I'd usually choose an object. I get another measurable speed
improvement by using an object in place of the closures.

My version of the code, with the above changes and a few smaller
refactorings, is here: https://gist.github.com/3885963

(I pasted your Xopen function in my version too since the go tool was
having trouble getting your package.)

I don't know how to verify that it actually still works correctly, but
my transformations were fairly mechanical so hopefully I haven't
screwed anything up.

- Evan

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