Avoid excessive reallocations in PolySet

[ochafik]

PolySet is a vector of polygons (vectors), so we can avoid a myriad of tiny (or big) reallocs by:

• Reserving the vector of polygons when it's easy (e.g. in binary STL import, primitives...)
• Requiring a polygon size arg in PolySet::append_poly to reserve the polygon's vector

sphere(10, $fn=1000); renders 1.18x faster with this!

And this one that does some actual CSG ops, still 1.13x faster (--enable=manifold --export-format=binstl):

sphere($fn=1000);
translate([0.5, 0, 0])
  sphere($fn=1000);

[kintel]

To merge once CI goes green