Add RFC 1238's unsafe_destructor_blind_to_params (UGEH) where needed.

I needed it in `RawVec`, `Vec`, and `TypedArena` for `rustc` to
bootstrap; but of course that alone was not sufficient for `make
check`.

Later I added `unsafe_destructor_blind_to_params` to collections, in
particular `LinkedList` and `RawTable` (the backing representation for
`HashMap` and `HashSet`), to get the regression tests exercising
cyclic structure from PR #27185 building.

----

Note that the feature is `dropck_parametricity` (which is not the same
as the attribute's name). We will almost certainly vary our strategy
here in the future, so it makes some sense to have a not-as-ugly name
for the feature gate. (The attribute name was deliberately selected to
be ugly looking.)

src/libstd/lib.rs

@@ -199,6 +199,11 @@
        test(no_crate_inject, attr(deny(warnings))),
        test(attr(allow(dead_code, deprecated, unused_variables, unused_mut))))]
 
+// SNAP 1af31d4
+#![allow(unused_features)]
+// SNAP 1af31d4
+#![allow(unused_attributes)]
+
 #![feature(alloc)]
 #![feature(allow_internal_unstable)]
 #![feature(associated_consts)]
@@ -241,6 +246,7 @@
 #![feature(unboxed_closures)]
 #![feature(unicode)]
 #![feature(unique)]
+#![feature(dropck_parametricity)]
 #![feature(unsafe_no_drop_flag, filling_drop)]
 #![feature(decode_utf16)]
 #![feature(unwind_attributes)]