Rename algorithms by what they return instead of their creator

Currently, many algorithms are not discoverable unless you know who created them. We should instead export more explicit function names, and add a type-based mechanism for dispatch with a reasonable default choice.
For instance,

function dijkstra(g, v)
    ...
end

function a_star(g, v)
    ...
end

might become

struct Dijkstra end
struct Astar end

function shortest_path(::Type{Dijkstra}, g, v)
    ...
end

function shortest_path(::Type{Astar}, g, v)
    ...
end

shortest_path(g, v) = shortest_path(Dijkstra(), g, v)

Start by locating the existing dijkstra and a_star entry points and inventorying the algorithms that would need explicit names. Define the scope of the shortest_path dispatch design, including its default choice; the work is done when the agreed algorithms expose the new naming and dispatch consistently.