allow doctests to refer to the pin-init crate

The next commit will name the pin-init crate from proc macros via
`::pin_init`. For this to work within tests of the pin-init crate
itself, it needs to be able to refer to itself via that name. Thus add
the required code for the name to be available.

Signed-off-by: Benno Lossin <[email protected]>

Author: y86-dev

src/lib.rs

@@ -300,6 +300,9 @@ mod alloc;
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub use alloc::InPlaceInit;
 
+#[allow(unused_extern_crates)]
+extern crate self as pin_init;
+
 /// Used to specify the pinning information of the fields of a struct.
 ///
 /// This is somewhat similar in purpose as