Return owned `String`s for onion message message types

Return owned `String`s for onion message message types

Returning a reference from a trait method is relatively difficult
to map in bindings and is currently handled by storing the object
in the trait instance, returning a reference to the local field.

This is fine when the object we're returning only needs to live as
long as the trait, but when it needs to be `'static` (as is the
case for onion message `msg_type`s), there's not really a good way
to map them at all.

Instead, here, condition on `#[cfg(c_bindings)]` we return a fully
owned `String`. This is obviously relatively less effecient, but
the extra allocation and `memcpy` isn't the end of the world,
especially given it should be released relatively quickly.

Note that this breaks doctests in with `c_bindings`.