Add a new crate `bitcoin-internals` to be used for internal code needed
by multiple soon-to-be-created crates.
Add the `write_err` macro to `bitcoin-internals`, nothing else.
This patch uses a `path` dependency which means `rust-bitcoin` cannot be
released in its current state, will need to be changed once we release
the `bitcoin-internals` crate on `crates.io`.
Create a directory `bitcoin` and move into it the following as is with
no code changes:
- src
- Cargo.toml
- contrib
- test_data
- examples
Then do:
- Add a workspace to the repository root directory.
- Add the newly created `bitcoin` crate to the workspace.
- Exclude `fuzz` and `embedded` crates from the workspace.
- Add a contrib/test.sh script that runs contrib/test.sh in each
sub-crate
- Fix the bitcoin/contrib/test.sh script