map_lgl should .f instead of .p

[hadley]

@lionel- what do you think? I think it's better to be consistent within this group of functions, rather providing useful additional behaviour that requires more explanation.

[lionel-]

hmm yes it made more sense when it was a single probe() function.

[lionel-]

A side effect is that now map_if(), keep() etc don't work with logical vectors. So we should probably reinstate probe() for internal use.

[hadley]

And we clearly need some unit tests for those possibilities too

[lionel-]

yup I'll do it.