[API] Fix several floating links issues & add Reroute.totalLinks (#815)

Resolves several issues with floating links.  Highlights:

- Caches floating links on slots, removing some loop checks (inefficient
/ does not scale)
- Simpler APIs
- Adds `Reroute.totalLinks` (regular and floating

src/LLink.ts

@@ -221,7 +221,7 @@ export class LLink implements LinkSegment, Serialisable<SerialisableLLink> {
 
     for (const reroute of reroutes) {
       reroute.linkIds.delete(this.id)
-      if (!keepReroutes && !reroute.linkIds.size && !reroute.floatingLinkIds.size) {
+      if (!keepReroutes && !reroute.totalLinks) {
         network.reroutes.delete(reroute.id)
       }
     }