Containers: add explicit StridedDimension conversion from/to external types

[mosra]

In addition to the implicit conversion. Desired use case is for types where we still want to have a conversion for convenience purposes, but the conversion is potentially dangerous (such as Magnum's Vector3i -> Size3D).

Fails to compile on MSVC 2019 and earlier due to the compiler thinking the two constructors / conversion operators are the same. 2022 works; all other compilers including GCC 4.8 work as well. The seemingly-conflicting overload is based on the answers in https://stackoverflow.com/questions/14600201/why-should-i-avoid-stdenable-if-in-function-signatures, there isn't any indication of MSVC being the odd one out.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.97%. Comparing base (90e0d7d) to head (4d7fa4e).
Report is 831 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #162   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files         136      136           
  Lines       11086    11089    +3     
=======================================
+ Hits        10861    10864    +3     
  Misses        225      225           

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