Extend core mesh API (Mesh, MeshMut, ...)

[LukasKalbertodt]

• Mutating methods
  • add_face/add_triangle
  • add_vertex
  • remove_face
  • remove_isolated_vertex
  • remove_vertex_with_adjacent_faces (and maybe find a shorter name): this can have a default implementation. This should probably be bounded by Self: FullAdj, right?
  • remove_all_faces/remove_all_vertices: improve documentation and fix semantics, especially regarding reuse of handles
  • split_face (1-to-n split)
  • flip_edge
  • split_edge_with_faces (although: maybe rename)
  • collapse_edge
  • Mutable iterator (this requires a bit more thinking, also about the exact semantics of Ref)
• Non-mutating
  • num_*
  • contains_*
  • valence_of_*?
  • is_manifold_vertex?