Optimise `count_temporal_edges` so it does not have to look at all edges in the graph

[ljeub-pometry]

Currently, one needs to use

graph.edges().explode().count()

to get the number of exploded edges (i.e., edge events) in the graph. This is both cumbersome and unnecessarily slow. We need to add a low-level method to the graph to get this count directly.

[miratepuffin]

Still needs to be optimised, but has a top level function as per #1149