rust: Add a log file rotation utility
This is ported to Rust from `mercurial/loggingutil.py`.
The "builder" pattern is used to make it visible at call sites what the two
numeric parameters mean. In Python they might simply by keyword arguments.
Differential Revision: https://phab.mercurial-scm.org/D10010
Hello, Foobar
[b'Hello, Fo, =;:<o', b'Hello, Bar']