builtin: Implement delattr

[corona10]

I've implemented delattr for builtin function.

[codecov-io]

Codecov Report

Merging #55 into master will increase coverage by 0.01%.
The diff coverage is 81.81%.

@@ Coverage Diff @@## master #55 +/- ## ========================================== + Coverage 66.42% 66.44% +0.01%  ========================================== Files 59 59 Lines 10368 10379 +11 ========================================== + Hits 6887 6896 +9 - Misses 2989 2990 +1 - Partials 492 493 +1

Impacted Files	Coverage Δ
builtin/builtin.go	79.62% <81.81%> (+0.04%)	⬆️

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[corona10]

@ncw
Please take a look.
This is a pretty easy PR then before. :)

I'd like to see that gpython could run stdlib in this year.
I expect that we must update some object structure.
And this will be a rough road.

[ncw]

Lgtm :-)