Cargo.toml: Add profile.checked-release (#1289)

Add a `--profile checked-release`, which is mostly `release`, but with
all checks enabled (e.x. `debug-assertions` and `overflow-checks`). This
doesn't need to absolute highest performance, so `opt-level = 2` is done
instead of `opt-level = 3`, and `codegen-units = 16` is used as well
(the normal `release` default, overridden by us to `codegen-units = 1`).

This is especially helpful when running tests locally under qemu, which
can be very slow if running the `opt-dev` profile. This could also be
used in CI instead of `opt-dev`, as `opt-level = 2` should help those
run quite a bit faster while still having all checks enabled.

Cargo.toml

@@ -62,3 +62,9 @@ panic = "abort"
 [profile.release-with-debug]
 inherits = "release"
 debug = "line-tables-only"
+
+[profile.checked-release]
+# Close to a release build, but with all checks enabled.
+inherits = "dev"
+opt-level = 2
+codegen-units = 16