Make Generator/GeneratorBase unmovable in addition to uncopyable
Make Generator/GeneratorBase unmovable in addition to uncopyable

Recent changes make using move semantics unsafe without extra work;
forbidding these in lieu of attempting to fix them.


Former-commit-id: dedca783c5bedd942e30ad70e95739b29cd5fef9
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide