Returns the 1-based index of the element nearest to value.
For ImplicitCoord, this is O(1) arithmetic.
For ExplicitCoord on numeric values, binary search if sorted,
linear scan otherwise.
Returns the 1-based index of the element nearest to value.
For ImplicitCoord, this is O(1) arithmetic.
For ExplicitCoord on numeric values, binary search if sorted,
linear scan otherwise.