Move task::task() to TaskBuilder::new()

The constructor for `TaskBuilder` is being changed to an associated
function called `new` for consistency with the rest of the standard
library.

Closes #13666

[breaking-change]
auto merge of #13675 : sfackler/rust/taskbuilder-new, r=alexcrichton

The constructor for `TaskBuilder` is being changed to an associated
function called `new` for consistency with the rest of the standard
library.

Closes #13666

[breaking-change]