Correction of setIntersection's description in the cheat sheet.

setIntersection's description was the same as setDifference's, which 
seemed like an error, I think I fixed it.
Extra doc and tests for hasElaborateAssign
Fixes Issue 8143 - Safe std.conv.to enum conversion
make nullable throw logic errors
Update std/traits.d
Re-enable unittest that is now working.
Refactor lockstep and simplify opApply string mixin generator.
Merge pull request #1123 from quickfur/cartprod_unittest

Re-enable unittest that is now working.
Merge pull request #1124 from AndrejMitrovic/RefactorLockstep

Refactor lockstep and simplify opApply string mixin generator.
Merge pull request #1103 from monarchdodra/Nullable

Fix Nullalble to throw logic errors instead of exceptions
Merge pull request #1080 from monarchdodra/postblit

Extra doc and tests for hasElaborateAssign
Merge pull request #1075 from torje/patch-1

Correction of setIntersection's description in the cheat sheet.
Merge pull request #897 from AndrejMitrovic/Fix_8143

Fix Issue 8143 - Safe std.conv.to enum conversion