Add support for `RefBox<'a, [T]>` and `RefBox<'a, str>`

[reem]

These are useful for conservatively serializing &str and &[T] then deserializing to Box<str> and Box<[T]>, which are analogous to String and Vec<u8>.

[reem]

cc rust-lang-deprecated/rustc-serialize#103

[TyOverby]

I'm not familiar with boxed unsized types. How does one go about making and using Box and Box<[T]>?

[reem]

Box<[T]> is constructed via either box static_array or Vec::into_boxed_slice. It has the same representation as &[T] but is heap allocated and owned.

[reem]

You could also solve this issue with specific RefVec<'a, T> and RefString<'a> types, if the boxed-unsized-types is weird.

[TyOverby]

#37 Will close this issue once I add tests.