Use adt_const_params for Scope and Semantics const generics

[nazar-pc]

This implements #607 and resolves #414 using adt_const_params.

Implementation has been on nightly for a long time without major known issues AFAIK. docs.rs already compiles docs with nightly toolchain, so should not be a problem there either.

This makes developer experience better with improved type safety. As a bonus ordering validity is now checked at compile time.

| between memory semantics options is technically possible with even more nightly features, but didn't seem worth it for now.

The changes are essentially mechanical.

[Firestar99]

Not considering this patch requiring a nightly feature, I think the patch looks good. I squashed in a small cleanup to deduplicate

-    const {
-        assert!(
-            SEMANTICS.has_valid_ordering(),
-            "at most one memory-ordering flag (ACQUIRE, RELEASE, ACQUIRE_RELEASE or \
-             SEQUENTIALLY_CONST) may be set"
-        );
-    }
-
+    const { SEMANTICS.assert_valid() }

Give me an hour or two, I have an idea I want to toy around with that may solve our stable vs nightly feature problem...