- use io-thread-pool git repo URL in stack.yaml extra-deps. no longer require
  local copy of that library.
- update docs
- add source URL in pypi metadata.

- use proper module name hierarchy.

This option allows combine big file when disk space is low.
it's not enabled by default to allow easy redownload/retry.

- switched to tinylog, it supports log level
- reorganize files to minimize code recompilation
- added --verbose for rd-api
- make FileStatus a data type
- update doc

sha1sum works fine with lazy ByteString.