Enable to stop in struct's IterFields method

[sairoutine]

As well as list and map, I would like to write the following the code by struct's iteration method.

var error err
str.IterFields(func(name string, value types.Value) bool {
    err = SomeMethod()
    if err != nil {
        return true
    }
    return false
})

if err != nil {
    return err
}

[sairoutine]

I finished the agreement.

[sairoutine]

It seems that it is not the matter of this pull request that fails the test of Travis.

[sairoutine]

@aboodman Thank you for your review. I finished fixing your pointed out. Please review again.

[arv]

Wasn't there some performance issues with assert.Equal?

[aboodman]

@arv is right - I forgot about that perf issue. I will land the version of the change that you originally posted.

Thanks for the patch!

[aboodman]

Actually the usage here is in a test file, so the perf is irrelevant. Landed as-is.

[sairoutine]

Thank you for your merging!