Complete refactor of how worker spawning works

Merge after #93, it contains those commits

This PR moves to use callr for worker spawning, which I think makes a few things more robust, particularly the way that callr arranges for child R sessions to be arranged similarly to the parent ones (library paths, etc). We already use callr for running tasks in a separate processes.

callr moves to be a core dependency, so removed all checks in tests that it is installed (this also has the effect that running in a separate process is always possible, which is nice)
the return value from rrq_worker_spawn is now a little R6 class that can be poked at; there's a bit of simplification through the tests and vignettes due to this

With this change in we barely use docopt, so I might look over that interface next

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05 :tada:

Comparison is base (91aa1f5) 99.89% compared to head (c9677f8) 99.95%.

:exclamation: Current head c9677f8 differs from pull request most recent head 0a4e41e. Consider uploading reports for the commit 0a4e41e to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #94 +/- ## ========================================== + Coverage 99.89% 99.95% +0.05% ========================================== Files 28 28 Lines 1973 2001 +28 ========================================== + Hits 1971 2000 +29 + Misses 2 1 -1 ``` | [Impacted Files](https://codecov.io/gh/mrc-ide/rrq/pull/94?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide) | Coverage Δ | | |---|---|---| | [R/utils.R](https://codecov.io/gh/mrc-ide/rrq/pull/94?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui91dGlscy5S) | `100.00% <ø> (+1.06%)` | :arrow_up: | | [R/worker\_spawn.R](https://codecov.io/gh/mrc-ide/rrq/pull/94?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide#diff-Ui93b3JrZXJfc3Bhd24uUg==) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mrc-ide)

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mrc-ide / rrq

Complete refactor of how worker spawning works #94

Codecov Report