Fix issue 8411 - add opCast!bool support for Duration.

[schveiguy]

Checking whether a duration is zero or not is a handy mechanism. Furthermore, it allows one to check if a duration is zero, and allows declaring a new duration variable at the same time:

if(d = calculateDuration())
{
   // do something with d
}

[dlang-bot]

Fix	Bugzilla	Description
✓	8411	core.time: No easy way to check if Duration is empty

[JackStouffer]

Returns:

[schveiguy]

Fixed

[schveiguy]

Not sure what the test error is, looks like a fluke segfault in unrelated std.parallelism, so I deprecated it. In any case, thank you all for the approvals. Can I get an automerge from one of you? 😉

[JackStouffer]

The green check mark vs the grey one shows who has merge permissions.

[schveiguy]

The green check mark vs the grey one shows who has merge permissions.

Ah, I forget you all don't have druntime cred. I will just push the button, don't think this is controversial.

[schveiguy]

Auto-merge toggled on

[WalterWaldron]

looks like a fluke segfault in unrelated std.parallelism,

My fix for that is dlang/phobos#5004 and it's been gathering dust for ~4 months.