Add transform algorithm

[jfirebaugh]

• For points, takes a points and a callable, and returns the result of calling the callable on the point.
• For collections, takes a collection and a callable, and returns a collection of the same collection type but with an element type that's the return type of calling transform on an element of the input collection, containing the result of calling transform on each element of the input. And if the collection supports reserve(), use it.
• For variants, takes a variant and a callable, and returns a variant whose option types consist of the return types of calling transform on each input variant option type.

Basically, it should support succinct definitions of other algorithms like:

template <class T>
auto project(const T& geometry) {
    return mapbox::geometry::transform(geometry, [] (const point<double>& p) -> point<int64_t> {
        // projection math
    });
}

[jfirebaugh]

The use case in geojson-vt-cpp requires transforming from the default geometry.hpp geometry variant type to a parallel variant type where some of the options are custom geometry types (inheriting from std::vector<...> with some additional members). A transform algorithm that's generic enough to support this scenario probably requires #26 plus some additional work.