upgrade rust toolchain to 1.70

This brings us closer to the downstream version. Windows link failures
appear with 1.72.1, possibly due to abrisco/cargo-bazel#173.

WORKSPACE

@@ -222,8 +222,8 @@ load("@rules_rust//rust:repositories.bzl", "rules_rust_dependencies", "rust_regi
 rules_rust_dependencies()
 
 rust_register_toolchains(
-    edition = "2018",
-    versions = ["1.66.0"],
+    edition = "2021",
+    versions = ["1.70.0"],
 )
 
 load("@rules_rust//crate_universe:repositories.bzl", "crate_universe_dependencies")