Expose TLV serialization/deserialization macros

[mariocynicys]

Macros found in lightning::utils::ser_macros could be pretty useful to LDK users.

Some use cases would be:

• Implementing custom messages containing TLVS.

The macro that interests me (and anyone implementing custom messages) is this one (could this possibly be made public?), but we should investigate if there are other serialization/deserialization macros which would be also useful to be made public.