Add : Box<_> or ::Box<_> type annotations to various places.

This is the kind of change that one is expected to need to make to
accommodate overloaded-`box`.

----

Note that this is not *all* of the changes necessary to accommodate
Issue 22181.  It is merely the subset of those cases where there was
already a let-binding in place that made it easy to add the necesasry
type ascription.

(For unnamed intermediate `Box` values, one must go down a different
route; `Box::new` is the option that maximizes portability, but has
potential inefficiency depending on whether the call is inlined.)

----

There is one place worth note, `run-pass/coerce-match.rs`, where I
used an ugly form of `Box<_>` type ascription where I would have
preferred to use `Box::new` to accommodate overloaded-`box`.  I
deliberately did not use `Box::new` here, because that is already done
in coerce-match-calls.rs.

----

Precursor for overloaded-`box` and placement-`in`; see Issue 22181.

src/liballoc/lib.rs

@@ -96,9 +96,15 @@ pub mod heap;
 
 // Primitive types using the heaps above
 
+// Need to conditionally define the mod from `boxed.rs` to avoid
+// duplicating the lang-items when building in test cfg; but also need
+// to allow code to have `use boxed::HEAP;`
+// and `use boxed::Box;` declarations.
 #[cfg(not(test))]
 pub mod boxed;
 #[cfg(test)]
+mod boxed { pub use std::boxed::{Box, HEAP}; }
+#[cfg(test)]
 mod boxed_test;
 pub mod arc;
 pub mod rc;