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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true