cmd/goexports,stdlib: parallelize symbols generation for stdlib

[sbinet]

Fixes #314.

[sbinet]

needs #311.

[ldez]

FYI I'm also working on the goexports command to add a smarter generation by platform, then I am interested in your change.

Could you improve the PR description.

Note: Linked PRs (or WIP PRs) are a pain to review, in this case, I would recommend waiting a bit before opening a new PR.

[ldez]

As I say, I'm working on the goexports command to add a smarter generation by platform.
Use parallelism is a good idea but I will change the core of goexports.
Then we will wait until I finished the smarter generation.

[sbinet]

ok. alternatively: we could make goexports smarter and recognize (like go list does) the "meta" package stdlib and have it automatically generate wrappers for all the packages that are in stdlib.

[ldez]

I do not understand why you close this PR. We are really interested in this PR.

[ldez]

If it's a problem, I can review this PR before creating mine, I try to do my best.