Trace task names in logging macros

[graydon]

Logging should emit task names, not just main. Probably task names should migrate to task-locals, when they exist, and logging should move to core.

[eholk]

Even just a task ID would be helpful until we have support for names.

The subject of task names came up recently in #2931.

[bstrie]

@brson, is this still relevant with the new runtime work?

[emberian]

/cc @bblum
Tasks seem to have names now, and I see them when a task fails.

[bblum]

They are not yet emitted during, for example, error! calls.

[catamorphism]

Just a bug, de-milestoning

[flaper87]

I'll work on this

[alexcrichton]

Updated the issue title to be more relevant to today's use-case

[reem]

Triage: attempting to fix in #19444.

[alexcrichton]

Fixed in #19444