Update generator to 0.7

Oh, generator 0.7 requires Rust 1.51.

error[E0658]: const generics are unstable
  --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/generator-0.7.0/src/gen_impl.rs:23:41
   |
23 | pub struct GeneratorObj<'a, A, T, const LOCAL: bool> {
   |                                         ^^^^^
   |
   = note: see issue #44580 <https://github.com/rust-lang/rust/issues/44580> for more information

EDIT: solved by #205

tokio-rs / loom

Update generator to 0.7 #203