Add sequenceIgnore and mapMIgnore to Validation - #365

Is the _ suffix sufficiently clear? Might this be better with a choose prefix? In particular, chooseM seems more appropriate than mapM_.

The underscore here basically means "ignore". AFAIK it's a common convention in F# and Haskell to use an underscore suffix in function names that represent the "value ignored" versions of such functions. The alternative names I can think of are mapMIgnore and sequenceIgnore

I don't see how renaming this function to chooseM is appropriate in this case. mapM_'s semantics is the same as in mapM. It's just that the possible success value will be dropped and we'll have unit instead.

I've typically seen the _ suffix used with lenses and optics, so that's probably what led to my confusion. I think using Ignore as a suffix would be clearer. Do you mind making that change?

Renamed the functions

@semyon2105 Thanks for the contribution!