Sustitute {callr} dependency

[gaborcsardi]

Cf. MangoTheCat#21

[gaborcsardi]

@lorenzwalthert I merged this, but it seems to break cyclocomp_package_dir(). Can you please take a look?

[lorenzwalthert]

ufff... Long time... Hopefully before the weekend.

[gaborcsardi]

Actually, never mind, I'll have to undo this, because if you load the package into the current R session once, then you won't be able to update and reload it without restarting R.

[lorenzwalthert]

Ok, so we can't get rid of the {callr} dependency?

[gaborcsardi]

Ok, so we can't get rid of the {callr} dependency?

Not without seriously crippling cyclocomp_package_dir().