bdk_core integration initial commit 🔥

We prepare the BDK repo for a major restructuring.

- database modules removed
- blockchain module removed
- minimal API changes.
- Many macros removed.
- no longer applicable examples removed.
- Much conditional compilation removed. Can compile with --all-features.
- delete verify module

.github/workflows/nightly_docs.yml

@@ -24,7 +24,7 @@ jobs:
       - name: Update toolchain
         run: rustup update
       - name: Build docs
-        run: cargo rustdoc --verbose --features=compiler,electrum,esplora,use-esplora-blocking,compact_filters,rpc,key-value-db,sqlite,all-keys,verify,hardware-signer -- --cfg docsrs -Dwarnings
+        run: cargo rustdoc --verbose --all-features -- --cfg docsrs -Dwarnings
       - name: Upload artifact
         uses: actions/upload-artifact@v2
         with: