Add `lightning-macros` crate

Add `lightning-macros` crate

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 `maybe_async` macros in the context of an `async KVStore`
implementation.