execution: Expose host and device policy classes via aliases

[stotko]

The host and device execution policies have been defined in #305 as aliases to the objects defined by thrust. However, the respective classes are not provided and must currently be retrieved via decltype and some type casting. Expose the respective classes via alias typedefs and use them to simplify the definition of the objects.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +0.12 :tada:

Comparison is base (7260dfe) 97.23% compared to head (c670dc2) 97.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #358 +/- ## ========================================== + Coverage 97.23% 97.35% +0.12% ========================================== Files 31 31 Lines 2421 2421 ========================================== + Hits 2354 2357 +3 + Misses 67 64 -3 ``` [see 1 file with indirect coverage changes](https://codecov.io/gh/stotko/stdgpu/pull/358/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Patrick+Stotko)

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