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 :-)