deque: Extend support for custom execution policies

[stotko]

Although all containers received support for custom execution_policys, this support is complete due to a left-over synchronous memcpy. Fortunately, this limitation has been resolved. Extend the support for deque accordingly to fill some of the remaining holes.

Partially addresses #351

[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.

Project coverage is 97.08%. Comparing base (3cb7082) to head (a2f4dd8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/stdgpu/impl/deque_detail.cuh	89.18%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #448      +/-   ##
==========================================
- Coverage   97.21%   97.08%   -0.14%     
==========================================
  Files          33       33              
  Lines        2624     2641      +17     
==========================================
+ Hits         2551     2564      +13     
- Misses         73       77       +4     

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