optionally allow subrefs instead of opstrings

[patch]

Optionally allow the first argument to be a subroutine ref instead of an operator string.

zip sub { "a: $_[0]; b: $_[1]" }, \@array1, \@array2;

Using zip, this provides no benefit over pairwise from List::MoreUtils:

# List::MoreUtils
pairwise { "a: $a; b: $b" } @array1, @array2;

However, it wouldn't hurt as an option, especially since we aren't using prototypes, and List::MoreUtils doesn't provide alternatives for cross and hyper.