Add FromSlicePtr

[KenxinKun]

This proposal implements the reverse of ToSlicePtr by using FilterMap internally to skip any nil pointer values in an input array of pointers and returning the corresponding array of values safely.

[samber]

I would use the same behavior of lo.FromPtr, that returns a zero value when a pointer is nil.

It can be combined with lo.Compact.

I suppose you can implement a lo.FromSlicePtrOr helper as well.

[samber]

Closing in favor of #217