rust-config: add support for default config items
Now that configitems.toml exists, we can read from it the default values for
all core config items.
We will add the devel-warning for use of undeclared config items in a later
patch when we're done adding the missing entries for `rhg`.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.