Add `lightning-macros` crate and drop `bdk_macros` dependency

[tnull]

Previously, we used the bdk_macros dependency for some simple proc macros in lightning-transaction-sync. However, post-1.0 BDK doesn't further maintain this crate and will at some point probably yank it together with the old bdk crate that was split up.

Here, we create a new crate for utility proc macros and ~~steal~~ add what we currently use (slightly modified for the latest syn version's API though). In the future we may want to expand this crate, e.g., for some further changes to the maybe_async macros in the context of an async KVStore implementation.