Simplify each arc separately.

Rather than build one large heap for all arcs, we build a heap for each arc separately and simplify one arc at a time.  A quick benchmark using Natural Earth’s 10m admin-0 data simplified at 1e-5 results in a 2x speedup with this change.  This is due to reduced peak memory usage and faster heap operations on the smaller heaps.

~~Note that this does modify the output slightly, because Visvalingam’s algorithm ensures the effective areas of points are always greater or equal to the previously-eliminated points, and this modification changes the previously-eliminated points, due to operating on the points from each arc separately.~~

~~However, this aspect is only really important on a per-arc basis, since it prevents initially important points from being considered unimportant later on, i.e. if  eliminated points result in a smaller effective area.~~