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]
This updates a number of ignore-test tests, and removes a few completely
outdated tests due to the feature being tested no longer being supported.
This brings a number of bench/shootout tests up to date so they're compiling
again. I make no claims to the performance of these benchmarks, it's just nice
to not have bitrotted code.
Closes#2604Closes#9407