Add d3.bisectBy(comparator).

Fixes #1766. Unlike d3.bisector(accessor), this allows you to define a bisector
that works in reverse order.

An awkward aspect of implementing bisection on top of a comparator is that it is
often the case that the sorted array contains objects (e.g., rows from a TSV),
while the search value is a primitive value (e.g., a number). Thus, you want to
apply an accessor to the array elements but not to the search value.

The solution here is to invoke the comparator deterministically: the first
argument is always an element from the array, and the second argument is always
the search value. This lets a comparator apply an accessor to array elements but
not to search values.