Fix Issue 10460 - auto ref for Union, SetDifference, SetSymmetricDifference

https://issues.dlang.org/show_bug.cgi?id=10460

The rearrangement of SetSymmetricDifference.front was needed since auto ref doesn't deduce correctly when there are multiple return statements (by design), so a single return statement with the ternary operator was used.
Fix regression 13393.

Caused by .save failing to save entire state, causing inconsistency in
.save'd objects.
Simplify unittest.
Make std.file.timeLastModified with two arguments safe
Merge pull request #2461 from fb-pja/bug10460

Fix Issue 10460 - auto ref Union, SetDifference, SetSymmetricDifference
Merge pull request #2473 from quickfur/issue13393

Fix regression 13393: cartesianProduct + joiner = runtime assertion failure
Merge pull request #2474 from tom-tan/safe-file-timeLastModified-2

Make std.file.timeLastModified with two arguments safe
Eliminate C-style array declarations.
Use auto instead of explicit array type.
Merge pull request #2475 from quickfur/issue13402

Eliminate C-style array declarations.