multiple, parallel calls to `$eval()`

[simonpcouch]

I've come across a few cases now where I want to run an eval across multiple solvers or scorers. We effectively parallelize within those calls, but users would just map() across them if there are multiple. Maybe the solution is to future_map, though maybe we could drop a nice UI around this.

Out of scope for the first release, but maybe worth revisiting later.

[simonpcouch]

I do think it's nice to have access to the Task objects natively in this sort of situation... I've recently just been spinning up different R sessions in Positron for different Task parameterizations.