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.

psuriana/Halide,psuriana/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,psuriana/Halide,kgnk/Halide,psuriana/Halide,kgnk/Halide