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. 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.
It may not be obvious why the condition in `push_bytes` module checks
for negation of 16 and 32 bit architectures rather than 64 bit. This
adds a comment about it being conservative.