impl<T> Read for UniquePtr<T> where ... Pin<&a mut T> : Read.

[anforowicz]

This commit implements forwarding of Read trait implementation from UniquePtr<T> to the pointee type. This is quite similar to how Box<T> also forwards - see
https://doc.rust-lang.org/std/boxed/struct.Box.html#impl-Read-for-Box%3CR%3E

Just as with Box<T>, the impl cannot be provided in the crate introducing T, because this violates the orphan rule. This means that before this commit a wrapper newtype would be required to work around the orphan rule - e.g.:

```
struct UniquePtrOfReadTrait(cxx::UniquePtr<ffi::ReadTrait>);
impl Read for UniquePtrOfReadTrait { … }
```

After this commit, one can provide an impl that works more directly with the C++ type T (the FFI will typically require passing self: Pin<&mut ffi::ReadTrait>):

```
impl<'a> Read for Pin<&'a mut ffi::ReadTrait> { … }
```

For a more specific motivating example, please see: https://docs.google.com/document/d/1EPn1Ss-hfOC6Ki_B5CC6GA_UFnY3TmoDLCP2HjP7bms

[anforowicz]

Can you PTAL?

Some notes:

• I wasn't sure if additional tests were required here. It seems to me that if the new impl compiles, then any additional tests wouldn't really provide extra value (maybe other than verifying that this really works end-to-end; OTOH I've manually done this kind of verification by checking that http://review.skia.org/889876 works fine).
• It may make sense to forward other std traits that Box<T> forwards, for example: Write, PartialEq, etc.
  • Please shout if you'd like me to:
    • Cover some additional traits in this PR (which traits?)
    • Cover some additional traits in follow-up PRs (which traits?)
  • I note that not all trait forwarding of Box<T> applies to cxx::UniquePtr<T>. For example:
    • From<T> takes T by value which is not possible for UniquePtrTargets (well, maybe except CxxString and CxxVector)

[dtolnay]

Thanks!

I am open to separate PRs for forwarding other traits (Write, PartialEq) as someone comes across a real use case for those.