fix(prim): export from the prelude, and document MST behaviour on disconnected graphs

[tachsin]

Two small things about prim, found while looking through the library.

prim is missing from the prelude

Every other algorithm is re-exported there; prim is not. So this does not compile:

use pathfinding::prelude::*;
let mst = prim(&edges);       // error[E0425]: cannot find function `prim` in this scope

and callers have to write pathfinding::undirected::prim::prim instead, while kruskal, sitting right next to it in src/undirected/, works from the prelude. I ran into this writing a throwaway test, which is how it turned up. One line in src/lib.rs.

The two MST functions disagree on disconnected graphs, silently

let edges = vec![(1, 2, 1), (3, 4, 1)];   // two components

prim(&edges)               // [(1, 2, 1)]                -- one component
kruskal(&edges).count()    // 2                          -- both components

prim grows outwards from the first endpoint of the first edge, so it spans only that component. kruskal sorts all edges and unions, so it spans every component and returns a forest.

Both behaviours are reasonable, and neither is a bug. The problem is that both doc comments just say "minimum-spanning-tree", which does not distinguish them, so a caller who swaps one for the other on a disconnected graph gets a quietly different answer. This documents the difference on both sides, with a runnable example on prim.

No behaviour change; docs and one re-export.