Add function to express list operations through 'NonEmpty'

[chshersh]

Add this function:

safe f = fmap f . nonEmpty

We export some functions to work with NonEmpty. But sometimes we want functions to work with [] and return Maybe. All such functions can be expressed through nonEmpty function. So it's nice to have such combinator. Also, safeHead can be safely removed.

[int-index]

Bad name.

[chshersh]

@int-index I was thinking on viaNonEmpty but this is too long. Propose better name.

[int-index]

Not too long.