ser: support LengthLimitedRead in impl_writeable_tlv_based_enum

When introducing the LengthLimitedRead trait in [1], for some
serialization macros we need some temporary workaround to
avoid that we have the following compilation error

error[E0277]: the trait bound `Bolt12Invoice: Readable` is not satisfied
    --> lightning/src/util/ser_macros.rs:1243:35
     |
1243 |                           Ok($st::$tuple_variant_name($crate::util::ser::Readable::read(reader)?))
     |                                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Readable` is not implemented for `Bolt12Invoice`
     |
    ::: lightning/src/events/mod.rs:2463:1
     |
2463 | / impl_writeable_tlv_based_enum_legacy!(PaidInvoice,
2464 | |     ;
2465 | |     (0, Bolt12Invoice),
2466 | |     (2, StaticInvoice)
2467 | | );
     | |_- in this macro invocation
     |
     = help: the following other types implement trait `Readable`:
               ()
               (A, B)
               (A, B, C)
               (A, B, C, D)
               (A, B, C, D, E)
               (A, B, C, D, E, F)
               (A, B, C, D, E, F, G)
               AnnouncementSigsState
             and 200 others
     = note: this error originates in the macro `impl_writeable_tlv_based_enum_legacy` (in Nightly builds, run with -Z macro-backtrace for more info)

This is a temporary workaround to avoid introducing a big refactoring right now.

[1] lightningdevkit#3640
Co-Developed-by: @valentinewallace
Suggested-by: @valentinewallace
Signed-off-by: Vincenzo Palazzo <[email protected]>

Author: vincenzopalazzo

lightning/src/util/ser_macros.rs

@@ -1200,7 +1200,7 @@ macro_rules! impl_writeable_tlv_based_enum {
 					$($tuple_variant_id => {
 						let length: $crate::util::ser::BigSize = $crate::util::ser::Readable::read(reader)?;
 						let mut s = $crate::util::ser::FixedLengthReader::new(reader, length.0);
-						let res = $crate::util::ser::Readable::read(&mut s)?;
+						let res = $crate::util::ser::LengthReadable::read_from_fixed_length_buffer(&mut s)?;
 						if s.bytes_remain() {
 							s.eat_remaining()?; // Return ShortRead if there's actually not enough bytes
 							return Err($crate::ln::msgs::DecodeError::InvalidValue);