Fix #9819: Allow access to named tuple's names

Adds a `fieldNames` alias to `std.typecons.Tuple` to return the user-supplied
names.  The actual fields are in an expression tuple, so the
user-supplied names are simply aliases, which previous to this PR, were
not accessible.

https://issues.dlang.org/show_bug.cgi?id=9819

Fix Issue 9819
Remove std.traits.unsigned as scheduled.
Move along some overdue deprecations.
There's no point in scheduling deprecations anymore.

deprecate no longer generates an error, so we might as well just
deprecate directly.
Update the Qt signals and slots link.
Merge pull request #2264 from ckakman/patch-1

Update the Qt signals and slots link.
Slight improvement to enforceEx's constraints and its documentation.
Workaround for issue# 12961.
Merge pull request #2260 from jmdavis/deprecated

Move some deprecations along.
Merge pull request #2232 from economicmodeling/fix_9819

Fix 9819: Allow access to named tuple's names.
Merge pull request #2265 from jmdavis/exception

Slight improvement to enforceEx's constraints and its documentation.