version 1.4.0

Changes since 1.3.3:

• The C implementation provides a CMakeLists.txt for callers who build with CMake. The CMake build is not yet stable, and callers should expect breaking changes in patch version updates. The "by hand" build will always continue to be supported and documented.
• b3sum supports the --seek flag, to set the starting position in the output stream.
• b3sum --check prints a summary of errors to stderr.
• Hash::as_bytes is const.
• Hash supports from_bytes, which is const.