Make LengthRead[able] public

A followup commit will require that LengthReadable be accessible from
outside the lightning crate, as it will be the default trait referenced
for TLV stream decoding.

Author: arik-so

lightning/src/util/ser.rs

@@ -344,8 +344,8 @@ where
 	fn read<R: Read>(reader: &mut R, params: P) -> Result<Self, DecodeError>;
 }
 
-/// A [`std::io::Read`] that also provides the total bytes available to be read.
-pub(crate) trait LengthRead: Read {
+/// A [`io::Read`] that also provides the total bytes available to be read.
+pub trait LengthRead: Read {
 	/// The total number of bytes available to be read.
 	fn total_bytes(&self) -> u64;
 }
@@ -361,9 +361,12 @@ where
 	fn read<R: LengthRead>(reader: &mut R, params: P) -> Result<Self, DecodeError>;
 }
 
-/// A trait that various higher-level LDK types implement allowing them to be read in
-/// from a [`Read`], requiring the implementer to provide the total length of the read.
-pub(crate) trait LengthReadable
+/// A trait that various LDK types implement allowing them to be read in from a [`Read`], requiring
+/// the implementer to provide the total length of the read.
+///
+/// Any type that implements [`Readable`] also automatically has a [`LengthReadable`]
+/// implementation, but some types, most notably onion packets, only implement [`LengthReadable`].
+pub trait LengthReadable
 where
 	Self: Sized,
 {