This PR updates docstrings and function signatures in order to complete
the transition from `Iter.Partial` to `Iter.Total` (extrinsically
terminating by default). It also deprecates `allowNontermination` and
adds `Iter.Total.atIdxSlow?`.
This PR adds infrastructure for parallel execution across Lean's tactic
monads.
- Add IO.waitAny' to Init/System/IO.lean for waiting on task completion
- Add `Lean.Elab.Task` with `asTask` utilities for `CoreM`, `MetaM`,
`TermElabM`, `TacticM`
- Add `Lean.Elab.Parallel` with parallel execution strategies:
* `par`/`par'` - collect results in original order
* `parIter`/`parIterGreedy` - iterate over results (original or
completion order) (also variants with a cancellation token)
* `parFirst` - return first successful result
This does *not* attempt to be a monad-polymorphic framework for
parallelism. It's intentionally hard-coded to the Lean tactic monads
which I need to work with. If there's desire to make this polymorphic,
hopefully that can be done separately.
