Fix new rustc #[macro_export] warning in doctests

rustc now warns any time a `#[macro_export]` is used inside a
function as it generates surprising results. Because doctests are
run inside implicit test functions this means any use of
`#[macro_export]` inside a doctest will now warn. We do this in
`lightning-custom-message` to demonstrated how to use the crate,
which now fails to compile.

Here we fix this by adding an `fn main() {}` to the doctest, which
causes doctests to be compiled as freestanding code rather than
inside a test function.

Note that while discussing this upstream it came up that rustc is
also planning on changing the way doctests are compiled to compile
an entire crate's doctests in one go rather than in separate
crates, causing doctests to have a shared namespace which may
generate future surprising outcomes.

Author: TheBlueMatt

lightning-custom-message/src/lib.rs

@@ -12,6 +12,7 @@
 //! `Foo` and `Bar` messages, and further composing it with a handler for `Baz` messages.
 //!
 //!```
+//! # fn main() {} // Avoid #[macro_export] generating an in-function warning
 //! # extern crate bitcoin;
 //! extern crate lightning;
 //! #[macro_use]
@@ -167,7 +168,6 @@
 //! #     }
 //! }
 //!
-//! # fn main() {
 //! // The first crate may define a handler composing `FooHandler` and `BarHandler` and export the
 //! // corresponding message type ids as a macro to use in further composition.
 //!
@@ -207,7 +207,6 @@
 //! macro_rules! foo_bar_baz_type_ids {
 //!     () => { foo_bar_type_ids!() | baz_type_id!() }
 //! }
-//! # }
 //!```
 //!
 //! [BOLT 1]: https://github.com/lightning/bolts/blob/master/01-messaging.md