vector: 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 vector accordingly to fill some of the remaining holes.

Partially addresses #351

[codecov]

Codecov Report

Attention: Patch coverage is 92.45283% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.93%. Comparing base (1fb525f) to head (f4dd289).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/stdgpu/impl/vector_detail.cuh	92.30%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #449      +/-   ##
==========================================
- Coverage   97.04%   96.93%   -0.12%     
==========================================
  Files          33       33              
  Lines        2641     2672      +31     
==========================================
+ Hits         2563     2590      +27     
- Misses         78       82       +4     

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