Fix some uses of begin/end, remove unnecessary includes

[K-ballo]

No description provided.

[taeguk]

@K-ballo I want to know what differences there are between std::begin and hpx::util::begin for Rng
Can the usage of std::begin occur bad things?

[K-ballo]

I want to know what differences there are between std::begin and hpx::util::begin for Rng

@taeguk As customization points, std::begin et.al. are meant to be called unqualified from generic code. Calling util::begin(rng) is effectively equivalent to using std::begin; begin(rng).

Can the usage of std::begin occur bad things?

Yes, since it's a different lookup and produces a different set of candidates, albeit it should be a rare occurrence in practice.

[hkaiser]

LGTM, thanks!