rust: Add a `ConfigValueParseError` variant to common errors Configuration files are parsed into sections of key/value pairs when they are read, but at that point values are still arbitrary bytes. Only when a value is accessed by various parts of the code do we know its expected type and syntax, so values are parsed at that point. Let’s make a new error type for this latter kind of parsing error, and add a variant to the common `HgError` so that most code can propagate it without much boilerplate. Differential Revision: https://phab.mercurial-scm.org/D10009

rust: Add config parsing support for more value types * Rust `str` (ASCII or UTF-8) * Integer * Byte quantities Differential Revision: https://phab.mercurial-scm.org/D10008

rust: Introduce a get_bytes_from_os_str utility function It does the same as get_bytes_from_path but takes an `OsStr` instead of a `Path`. The implementation is the same so using either ends up correct but the function name suggests it’s not. Differential Revision: https://phab.mercurial-scm.org/D10007