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.
std.typecons.Proxy needs to import std.traits.isArray

... because it is a mixin template which is evaluated at the place of
instantiation, where isArray is likely not available.
Make std.file.setTimes safe
Tweak enum table
Make std.file.timeLastModified safe
Partially evaluates -> Partially applies
Prevent definition of non-disabled default constructors in template structs
Merge pull request #2468 from 9rnsr/fix13311

Prevent definition of non-disabled default constructors in template structs
Merge pull request #2464 from monarchdodra/decodeTT

Tweak enum tables in std/utf.d
Merge pull request #2467 from ntrel/partial

Fix docs for std.functional.partial
fix Issue 13380 - Conflict with std.typecons.Identity and std.traits.Identity
Merge pull request #2470 from CyberShadow/fix13380

fix Issue 13380 - Conflict with std.typecons.Identity and std.traits.Identity
Merge pull request #2465 from tom-tan/safe-file-setTimes

Make std.file.setTimes safe
Remove void initializers
Merge pull request #2463 from schuetzm/fix-proxy-traits-isarray

std.typecons.Proxy needs to import std.traits.isArray
Merge pull request #2466 from tom-tan/safe-file-timeLastModified

Make std.file.timeLastModified(in char[]) safe
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