hg-core: add basic config module The config module exposes a `Config` struct, unused for now. It only reads the config file local to the repository, but handles all valid patterns and includes/unsets. It is structured in layers instead of erasing by reverse order of precedence, allowing us to transparently know more about the config for debugging purposes, and potentially other things I haven't thought about yet. This change also introduces `format_bytes!` to `hg-core`. Differential Revision: https://phab.mercurial-scm.org/D9408