Replace UpdateSolid::add_shell with add_shells

This is more flexible and follows the convention of other, similar
methods of the operations API.

crates/fj-core/src/operations/build/solid.rs

@@ -24,7 +24,7 @@ pub trait BuildSolid {
         services: &mut Services,
     ) -> Tetrahedron {
         let shell = Shell::tetrahedron(points, services).insert(services);
-        let solid = Solid::empty().add_shell(shell.shell.clone());
+        let solid = Solid::empty().add_shells([shell.shell.clone()]);
 
         Tetrahedron { solid, shell }
     }

crates/fj-core/src/operations/update/solid.rs

@@ -6,12 +6,18 @@ use crate::{
 /// Update a [`Solid`]
 pub trait UpdateSolid {
     /// Add a shell to the solid
-    fn add_shell(&self, shell: Handle<Shell>) -> Solid;
+    fn add_shells(
+        &self,
+        shells: impl IntoIterator<Item = Handle<Shell>>,
+    ) -> Solid;
 }
 
 impl UpdateSolid for Solid {
-    fn add_shell(&self, shell: Handle<Shell>) -> Solid {
-        let shells = self.shells().cloned().chain([shell]);
+    fn add_shells(
+        &self,
+        shells: impl IntoIterator<Item = Handle<Shell>>,
+    ) -> Solid {
+        let shells = self.shells().cloned().chain(shells);
         Solid::new(shells)
     }
 }