As we did for the `bitcoin` crate, remove attribution from all files in
the `internals` crate.
While we are at it add an SPDX line to the few files missing it, whether
this license nonsense is even needed is left as an argument for another
day.
Justification:
Currently we have a mishmash of attribution lines accompanying the SPDX
identifier. These lines are basically meaningless because:
- The date is often wrong
- The original author attributed is not the only contributor to a file
- The term "rust bitcoin developers" is basically just noise
Just remove all the attribution lines and be done with it.
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`.
Tobin C. Harding